<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2021 Workiva-->
<!--r:f4841171-a78a-4601-9818-8cb82493aa8f,g:6c378993-31d1-443a-9853-0f9ff53d4e3d-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.comcast.com/role/CoverPage" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#CoverPage"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/CoverPage" xlink:type="extended" id="i1c767076ca5443bbb4b8cfe652bc7027_CoverPage">
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:href="cmcsa-20210930.xsd#cmcsa_DocumentAndEntityInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_7bbcea93-aedf-4f83-ae3a-d1aa56c4b29a" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_DocumentType_7bbcea93-aedf-4f83-ae3a-d1aa56c4b29a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_616a0445-0871-4cd1-9dde-710abaf9c4b0" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_DocumentQuarterlyReport_616a0445-0871-4cd1-9dde-710abaf9c4b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_661b6d2c-464f-4cb0-8270-4a3e70708d5c" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_DocumentPeriodEndDate_661b6d2c-464f-4cb0-8270-4a3e70708d5c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_620b1ad2-91ba-42cd-a222-87f9408d1897" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_DocumentTransitionReport_620b1ad2-91ba-42cd-a222-87f9408d1897" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_90bbb7b0-895e-4e01-9950-6c89f21b9544" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityFileNumber_90bbb7b0-895e-4e01-9950-6c89f21b9544" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_1282c8f4-078a-4f61-b65e-acce0820dc89" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityRegistrantName_1282c8f4-078a-4f61-b65e-acce0820dc89" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_77008d24-b0f2-4dec-b4db-d0dceffb8134" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityTaxIdentificationNumber_77008d24-b0f2-4dec-b4db-d0dceffb8134" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_b84ad1f3-c5f1-483b-98a8-0129a0c6ad3c" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityIncorporationStateCountryCode_b84ad1f3-c5f1-483b-98a8-0129a0c6ad3c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_6ffac82e-14ee-43df-8f3d-b8d3202c1d9f" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityAddressAddressLine1_6ffac82e-14ee-43df-8f3d-b8d3202c1d9f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_d9b2a519-e572-41a1-8a0b-690fcc2848ee" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityAddressCityOrTown_d9b2a519-e572-41a1-8a0b-690fcc2848ee" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_6c9d6cf1-a901-4fff-aec9-b397a54e1cb3" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityAddressStateOrProvince_6c9d6cf1-a901-4fff-aec9-b397a54e1cb3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_1ec6cd9b-e300-476f-b0c1-c45532a3c7d7" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityAddressPostalZipCode_1ec6cd9b-e300-476f-b0c1-c45532a3c7d7" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_2a71f091-d3ba-4eca-9204-2fd2d3064b9c" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_CityAreaCode_2a71f091-d3ba-4eca-9204-2fd2d3064b9c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_a92b06f5-4bc5-4c5e-8430-a5aaceabbd67" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_LocalPhoneNumber_a92b06f5-4bc5-4c5e-8430-a5aaceabbd67" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_6c468772-7f3a-4856-af56-ae3493cf2a2e" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_Security12bTitle_6c468772-7f3a-4856-af56-ae3493cf2a2e" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_1592822f-1453-4380-965e-781d1a4ce9fb" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_TradingSymbol_1592822f-1453-4380-965e-781d1a4ce9fb" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_d081a5b9-f4c9-4215-9a6c-8c943f6e9d39" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_SecurityExchangeName_d081a5b9-f4c9-4215-9a6c-8c943f6e9d39" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_5f282130-5d67-4a1a-8279-c37abd8caeb6" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityCurrentReportingStatus_5f282130-5d67-4a1a-8279-c37abd8caeb6" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_3fe5be16-2440-4cfe-a8ca-d40f2612588b" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityInteractiveDataCurrent_3fe5be16-2440-4cfe-a8ca-d40f2612588b" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_7f33c91c-42b2-4698-afd1-807dce244230" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityFilerCategory_7f33c91c-42b2-4698-afd1-807dce244230" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_acd147df-daf6-42ef-9a18-59186d385312" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntitySmallBusiness_acd147df-daf6-42ef-9a18-59186d385312" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_cb0a1901-fdd7-4fee-be63-35f025efd795" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityEmergingGrowthCompany_cb0a1901-fdd7-4fee-be63-35f025efd795" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_14523115-3855-48c9-9b26-3d77e0d7e96f" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityShellCompany_14523115-3855-48c9-9b26-3d77e0d7e96f" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_d2c9cd2e-0777-44de-bab3-f5f792607d4c" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_d2c9cd2e-0777-44de-bab3-f5f792607d4c" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_2868e838-ee9c-4be3-b61a-1f90b80aea4b" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_DocumentFiscalPeriodFocus_2868e838-ee9c-4be3-b61a-1f90b80aea4b" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_f3486e3b-6790-4f74-904b-ae557b144972" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_AmendmentFlag_f3486e3b-6790-4f74-904b-ae557b144972" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_4ad4ed2f-7d39-4edd-a344-61a38028d7f6" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_DocumentFiscalYearFocus_4ad4ed2f-7d39-4edd-a344-61a38028d7f6" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_d92ac0c5-0696-4cd4-bc32-bf08ab27c544" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_CurrentFiscalYearEndDate_d92ac0c5-0696-4cd4-bc32-bf08ab27c544" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_79fe0c8b-bcd0-4747-bbf3-58fe912c8471" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_dei_EntityCentralIndexKey_79fe0c8b-bcd0-4747-bbf3-58fe912c8471" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_DocumentAndEntityInformationTable_f6d2d89b-b7bb-4cff-9737-a51b27ac0475" xlink:href="cmcsa-20210930.xsd#cmcsa_DocumentAndEntityInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_cmcsa_DocumentAndEntityInformationLineItems_8568a9bd-e430-4228-a1cc-0f46536a0ced" xlink:to="loc_cmcsa_DocumentAndEntityInformationTable_f6d2d89b-b7bb-4cff-9737-a51b27ac0475" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_dad69c40-0d6c-454b-95ad-4c756483a714" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_cmcsa_DocumentAndEntityInformationTable_f6d2d89b-b7bb-4cff-9737-a51b27ac0475" xlink:to="loc_us-gaap_StatementClassOfStockAxis_dad69c40-0d6c-454b-95ad-4c756483a714" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_dad69c40-0d6c-454b-95ad-4c756483a714_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_dad69c40-0d6c-454b-95ad-4c756483a714" xlink:to="loc_us-gaap_ClassOfStockDomain_dad69c40-0d6c-454b-95ad-4c756483a714_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_dad69c40-0d6c-454b-95ad-4c756483a714" xlink:to="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_35345ead-5116-431e-bb17-335098516ca9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_us-gaap_CommonClassAMember_35345ead-5116-431e-bb17-335098516ca9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes0000PercentDue2026Member_f9226408-4105-4698-b747-deb2c8aedf6c" xlink:href="cmcsa-20210930.xsd#cmcsa_Notes0000PercentDue2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_Notes0000PercentDue2026Member_f9226408-4105-4698-b747-deb2c8aedf6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes0.250percentDue2027Member_ce36a8be-71a9-4518-aabc-505d37f43480" xlink:href="cmcsa-20210930.xsd#cmcsa_Notes0.250percentDue2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_Notes0.250percentDue2027Member_ce36a8be-71a9-4518-aabc-505d37f43480" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes1.500percentDue2029Member_8b0a101e-c56b-4bf2-8d20-1937cec826ab" xlink:href="cmcsa-20210930.xsd#cmcsa_Notes1.500percentDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_Notes1.500percentDue2029Member_8b0a101e-c56b-4bf2-8d20-1937cec826ab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes00250PercentDue2029Member_f3f30088-4778-49ad-abf7-22de215f93b6" xlink:href="cmcsa-20210930.xsd#cmcsa_Notes00250PercentDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_Notes00250PercentDue2029Member_f3f30088-4778-49ad-abf7-22de215f93b6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes0.750percentdue2032Member_46b609e7-3bf1-41fa-9d14-7b98cc672f7e" xlink:href="cmcsa-20210930.xsd#cmcsa_Notes0.750percentdue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_Notes0.750percentdue2032Member_46b609e7-3bf1-41fa-9d14-7b98cc672f7e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes1.875percentdue2036Member_4351ac5e-ea04-4621-8d56-92979dc09fb7" xlink:href="cmcsa-20210930.xsd#cmcsa_Notes1.875percentdue2036Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_Notes1.875percentdue2036Member_4351ac5e-ea04-4621-8d56-92979dc09fb7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes1.250percentdue2040Member_2785813c-a5d3-49a2-976c-76166439c666" xlink:href="cmcsa-20210930.xsd#cmcsa_Notes1.250percentdue2040Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_Notes1.250percentdue2040Member_2785813c-a5d3-49a2-976c-76166439c666" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_GuaranteedNotes9.455PercentDue2022Member_074b4712-c14e-46d0-b342-af4338b2d623" xlink:href="cmcsa-20210930.xsd#cmcsa_GuaranteedNotes9.455PercentDue2022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_GuaranteedNotes9.455PercentDue2022Member_074b4712-c14e-46d0-b342-af4338b2d623" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes5.50PercentDue2029Member_87a7d8eb-7731-4bf7-adfd-a698b13659bf" xlink:href="cmcsa-20210930.xsd#cmcsa_Notes5.50PercentDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_Notes5.50PercentDue2029Member_87a7d8eb-7731-4bf7-adfd-a698b13659bf" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ExchangeableSubordinatedDebentures2.0PercentDue2029Member_4935c57c-84f3-488f-82a5-22833ab9df99" xlink:href="cmcsa-20210930.xsd#cmcsa_ExchangeableSubordinatedDebentures2.0PercentDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_cmcsa_ExchangeableSubordinatedDebentures2.0PercentDue2029Member_4935c57c-84f3-488f-82a5-22833ab9df99" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_aba95752-7c92-4b6f-bc2c-203f1c128bf8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_884d653c-3c69-4648-9823-f809e03b5f0d" xlink:to="loc_us-gaap_CommonClassBMember_aba95752-7c92-4b6f-bc2c-203f1c128bf8" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/CondensedConsolidatedBalanceSheetUnaudited" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#CondensedConsolidatedBalanceSheetUnaudited"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/CondensedConsolidatedBalanceSheetUnaudited" xlink:type="extended" id="i97b0845b26b24cc68297ebe220d1ebdd_CondensedConsolidatedBalanceSheetUnaudited">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_72448e6d-8059-44e6-a0e2-24c57289bbb2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_72448e6d-8059-44e6-a0e2-24c57289bbb2" xlink:to="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_0fc1a2b5-8ec1-43aa-b0df-a4d80b928171" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_us-gaap_AssetsCurrentAbstract_0fc1a2b5-8ec1-43aa-b0df-a4d80b928171" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_718c47f8-6a2f-41e4-b070-51c68a0a256b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_0fc1a2b5-8ec1-43aa-b0df-a4d80b928171" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_718c47f8-6a2f-41e4-b070-51c68a0a256b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_6b8acc78-8bf5-4884-8a76-fee19fde2dfd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_0fc1a2b5-8ec1-43aa-b0df-a4d80b928171" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_6b8acc78-8bf5-4884-8a76-fee19fde2dfd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsCurrent_fa1ce80a-2dc8-4f9e-a5c7-8400d7ac1d8f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_0fc1a2b5-8ec1-43aa-b0df-a4d80b928171" xlink:to="loc_us-gaap_OtherAssetsCurrent_fa1ce80a-2dc8-4f9e-a5c7-8400d7ac1d8f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_0b3e141a-2440-48a5-8e68-77b90fc24bc9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_0fc1a2b5-8ec1-43aa-b0df-a4d80b928171" xlink:to="loc_us-gaap_AssetsCurrent_0b3e141a-2440-48a5-8e68-77b90fc24bc9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_FilmMonetizedOnItsOwnAndEntertainmentLicenseAgreementForProgramMaterialIncludingSportsRightsAdvancesCapitalizedCostsNoncurrent_6ac7f718-5a90-40a2-a00f-e2f94bf7ba19" xlink:href="cmcsa-20210930.xsd#cmcsa_FilmMonetizedOnItsOwnAndEntertainmentLicenseAgreementForProgramMaterialIncludingSportsRightsAdvancesCapitalizedCostsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_cmcsa_FilmMonetizedOnItsOwnAndEntertainmentLicenseAgreementForProgramMaterialIncludingSportsRightsAdvancesCapitalizedCostsNoncurrent_6ac7f718-5a90-40a2-a00f-e2f94bf7ba19" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermInvestments_8a0ad4d9-c309-47cb-bfb6-112da5bad163" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_us-gaap_LongTermInvestments_8a0ad4d9-c309-47cb-bfb6-112da5bad163" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_InvestmentSecuringCollateralizedObligation_9df09d4d-580f-4ab2-abcd-d8ec3530a1d7" xlink:href="cmcsa-20210930.xsd#cmcsa_InvestmentSecuringCollateralizedObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_cmcsa_InvestmentSecuringCollateralizedObligation_9df09d4d-580f-4ab2-abcd-d8ec3530a1d7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_719fc7d5-5e14-4d52-aed4-cfd78b5fccf5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_719fc7d5-5e14-4d52-aed4-cfd78b5fccf5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_f5717a83-d5f4-4293-af10-61b98dd62186" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_us-gaap_Goodwill_f5717a83-d5f4-4293-af10-61b98dd62186" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedFranchiseRights_1acceb4e-c554-4a69-bf6b-61ddf043f378" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IndefiniteLivedFranchiseRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_us-gaap_IndefiniteLivedFranchiseRights_1acceb4e-c554-4a69-bf6b-61ddf043f378" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsNet_a6f8d84c-f938-48bb-9921-2446a4e2a97c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_us-gaap_OtherIntangibleAssetsNet_a6f8d84c-f938-48bb-9921-2446a4e2a97c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_8c466d3d-88dc-4cb6-b163-f98a52e6f49d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_8c466d3d-88dc-4cb6-b163-f98a52e6f49d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_5f8b85ba-b922-43d6-8c76-e5a9beacf3e9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_59b6a0f3-0f1a-4d13-8396-a11b5639c9e1" xlink:to="loc_us-gaap_Assets_5f8b85ba-b922-43d6-8c76-e5a9beacf3e9" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_72448e6d-8059-44e6-a0e2-24c57289bbb2" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_33549527-6995-47b4-8967-7a53d8e75fd0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_33549527-6995-47b4-8967-7a53d8e75fd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_6c45ad24-0487-4635-9749-cd946f821a22" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_33549527-6995-47b4-8967-7a53d8e75fd0" xlink:to="loc_us-gaap_AccountsPayableCurrent_6c45ad24-0487-4635-9749-cd946f821a22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_AccruedParticipationsAndResidualsCurrent_f83072b3-949b-4d30-82e7-bb425689cc75" xlink:href="cmcsa-20210930.xsd#cmcsa_AccruedParticipationsAndResidualsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_33549527-6995-47b4-8967-7a53d8e75fd0" xlink:to="loc_cmcsa_AccruedParticipationsAndResidualsCurrent_f83072b3-949b-4d30-82e7-bb425689cc75" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredRevenueCurrent_144763f2-c5b6-4d03-ad85-b9ddf29b6391" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredRevenueCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_33549527-6995-47b4-8967-7a53d8e75fd0" xlink:to="loc_us-gaap_DeferredRevenueCurrent_144763f2-c5b6-4d03-ad85-b9ddf29b6391" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_1d5a472b-5499-4628-8bf4-d5bf56a81979" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_33549527-6995-47b4-8967-7a53d8e75fd0" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_1d5a472b-5499-4628-8bf4-d5bf56a81979" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtCurrent_af851cca-5cee-4625-a3f2-386de25796ec" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_33549527-6995-47b4-8967-7a53d8e75fd0" xlink:to="loc_us-gaap_DebtCurrent_af851cca-5cee-4625-a3f2-386de25796ec" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_4d27e74d-9e6e-4442-8e0e-3134f61cedca" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_33549527-6995-47b4-8967-7a53d8e75fd0" xlink:to="loc_us-gaap_LiabilitiesCurrent_4d27e74d-9e6e-4442-8e0e-3134f61cedca" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_f295014b-59fa-4d70-b3cc-d97ff66a975e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_f295014b-59fa-4d70-b3cc-d97ff66a975e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CollateralizedObligation_9e27af90-2842-49a4-aaf9-e05d055ebb9a" xlink:href="cmcsa-20210930.xsd#cmcsa_CollateralizedObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_cmcsa_CollateralizedObligation_9e27af90-2842-49a4-aaf9-e05d055ebb9a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_00ec5a59-8cd9-4b90-a237-024268b6049a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_00ec5a59-8cd9-4b90-a237-024268b6049a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_8cb67b51-2aa6-4d36-87a2-2ad596c08573" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_8cb67b51-2aa6-4d36-87a2-2ad596c08573" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_ccd5bf5a-3e15-4009-a417-30de7c3b941e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_us-gaap_CommitmentsAndContingencies_ccd5bf5a-3e15-4009-a417-30de7c3b941e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_9eeafcd9-cdaf-4b01-835e-c37f6b1d48dd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_9eeafcd9-cdaf-4b01-835e-c37f6b1d48dd" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_27a371db-cb16-49f1-bf41-e8c33d8647d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_PreferredStockValue_27a371db-cb16-49f1-bf41-e8c33d8647d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_a497cfd4-40e1-40dd-8b55-e99b4b4a91fb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_CommonStockValue_a497cfd4-40e1-40dd-8b55-e99b4b4a91fb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_a2d83cb6-6a12-4ae7-9151-d9d38e99cd70" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_AdditionalPaidInCapital_a2d83cb6-6a12-4ae7-9151-d9d38e99cd70" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_2fac8ac9-494b-4dfc-abe8-0335574f97ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_2fac8ac9-494b-4dfc-abe8-0335574f97ff" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValue_9c61a22c-1964-4e6b-bab6-7afdf8b727aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TreasuryStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_TreasuryStockValue_9c61a22c-1964-4e6b-bab6-7afdf8b727aa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_b13c5d12-6d3f-4a36-bdcb-1d4a91ad4b30" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_b13c5d12-6d3f-4a36-bdcb-1d4a91ad4b30" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_dd967922-08ec-439c-9a4f-5a1593c65d60" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_StockholdersEquity_dd967922-08ec-439c-9a4f-5a1593c65d60" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterest_8e751feb-82f2-474b-95a5-8927538b5f3d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MinorityInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_MinorityInterest_8e751feb-82f2-474b-95a5-8927538b5f3d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_2d3c7315-db8b-47b6-b009-0b77574420b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_b77bab7d-2d10-438e-a0b4-708d40e3b7d1" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_2d3c7315-db8b-47b6-b009-0b77574420b8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_4f92ef97-9630-4f05-b960-525873bde937" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_f2fee34f-fe5e-4cb4-a703-673afd14d8ed" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_4f92ef97-9630-4f05-b960-525873bde937" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_c55fac5e-8fe5-4bb8-a769-31ab3922da0c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_72448e6d-8059-44e6-a0e2-24c57289bbb2" xlink:to="loc_us-gaap_StatementTable_c55fac5e-8fe5-4bb8-a769-31ab3922da0c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_2df13d35-fb2f-4583-b224-e0ab37b000ad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_c55fac5e-8fe5-4bb8-a769-31ab3922da0c" xlink:to="loc_us-gaap_StatementClassOfStockAxis_2df13d35-fb2f-4583-b224-e0ab37b000ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_2df13d35-fb2f-4583-b224-e0ab37b000ad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_2df13d35-fb2f-4583-b224-e0ab37b000ad" xlink:to="loc_us-gaap_ClassOfStockDomain_2df13d35-fb2f-4583-b224-e0ab37b000ad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b756a091-b539-4d08-b346-0f343df91e2b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_2df13d35-fb2f-4583-b224-e0ab37b000ad" xlink:to="loc_us-gaap_ClassOfStockDomain_b756a091-b539-4d08-b346-0f343df91e2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_cec9f30f-8f26-493a-8789-4583d5a3b378" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_b756a091-b539-4d08-b346-0f343df91e2b" xlink:to="loc_us-gaap_CommonClassAMember_cec9f30f-8f26-493a-8789-4583d5a3b378" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_8bf9afeb-5b92-4539-8da1-ae039b951953" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_b756a091-b539-4d08-b346-0f343df91e2b" xlink:to="loc_us-gaap_CommonClassBMember_8bf9afeb-5b92-4539-8da1-ae039b951953" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/CondensedConsolidatedBalanceSheetUnauditedParenthetical" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#CondensedConsolidatedBalanceSheetUnauditedParenthetical"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/CondensedConsolidatedBalanceSheetUnauditedParenthetical" xlink:type="extended" id="i910011cbd7c94985b8e4b43f2ec7b3af_CondensedConsolidatedBalanceSheetUnauditedParenthetical">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_5ba7167f-c38e-4372-8497-7aa68a25c523" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_5ba7167f-c38e-4372-8497-7aa68a25c523" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_cbdb4576-2f3c-4c00-b443-039b23939613" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_cbdb4576-2f3c-4c00-b443-039b23939613" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_d2f4e422-fa13-40e3-9c9f-8d4517dbb55b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_d2f4e422-fa13-40e3-9c9f-8d4517dbb55b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_3cb8c01e-a729-4bdd-94b5-1f05d17545b2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_PreferredStockSharesIssued_3cb8c01e-a729-4bdd-94b5-1f05d17545b2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_32842a42-dae4-4e07-9578-8798dfb254df" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_32842a42-dae4-4e07-9578-8798dfb254df" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_c5e42ed5-ba4e-456a-94fe-b374d034ad9b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_c5e42ed5-ba4e-456a-94fe-b374d034ad9b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_41de160f-657a-4289-bfd0-e48de93d7ea8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_CommonStockSharesIssued_41de160f-657a-4289-bfd0-e48de93d7ea8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_31f6c38e-d3cb-4ad1-82fd-fc7a4f9460aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_31f6c38e-d3cb-4ad1-82fd-fc7a4f9460aa" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockShares_71850002-c476-4e0e-bc3f-14389d90c488" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TreasuryStockShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_TreasuryStockShares_71850002-c476-4e0e-bc3f-14389d90c488" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_124fe273-b8d5-47ab-8f4d-56b958ee1ea6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_8713df55-d094-4ad2-83db-3e9d560e6155" xlink:to="loc_us-gaap_StatementTable_124fe273-b8d5-47ab-8f4d-56b958ee1ea6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_1e2e5b0e-d00d-4fac-b745-d581706cef41" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_124fe273-b8d5-47ab-8f4d-56b958ee1ea6" xlink:to="loc_us-gaap_StatementClassOfStockAxis_1e2e5b0e-d00d-4fac-b745-d581706cef41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_1e2e5b0e-d00d-4fac-b745-d581706cef41_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1e2e5b0e-d00d-4fac-b745-d581706cef41" xlink:to="loc_us-gaap_ClassOfStockDomain_1e2e5b0e-d00d-4fac-b745-d581706cef41_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_eb3d0ed0-4e84-425e-b349-42541cd98310" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_1e2e5b0e-d00d-4fac-b745-d581706cef41" xlink:to="loc_us-gaap_ClassOfStockDomain_eb3d0ed0-4e84-425e-b349-42541cd98310" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_520a212b-ddab-4a3a-a101-bf745f194cf8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_eb3d0ed0-4e84-425e-b349-42541cd98310" xlink:to="loc_us-gaap_CommonClassAMember_520a212b-ddab-4a3a-a101-bf745f194cf8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_f7f53582-ebb7-4dc8-941e-2e57f7956d3b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_eb3d0ed0-4e84-425e-b349-42541cd98310" xlink:to="loc_us-gaap_CommonClassBMember_f7f53582-ebb7-4dc8-941e-2e57f7956d3b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/CondensedConsolidatedStatementofChangesinEquityUnaudited" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#CondensedConsolidatedStatementofChangesinEquityUnaudited"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/CondensedConsolidatedStatementofChangesinEquityUnaudited" xlink:type="extended" id="id7383dad7e5d454282018fc6841f354c_CondensedConsolidatedStatementofChangesinEquityUnaudited">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_a31cb41a-3a24-45d0-8db7-0a63d68c9f79" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_9cda9a7f-1c82-450b-a221-8778a5b4051f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a31cb41a-3a24-45d0-8db7-0a63d68c9f79" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_9cda9a7f-1c82-450b-a221-8778a5b4051f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_691d3552-52c3-4a95-aa12-39fa7e4e6539" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_9cda9a7f-1c82-450b-a221-8778a5b4051f" xlink:to="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_691d3552-52c3-4a95-aa12-39fa7e4e6539" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_TemporaryEquityRedemptionOfPreferredStock_42232d11-cc85-40ae-9a35-cacff503879f" xlink:href="cmcsa-20210930.xsd#cmcsa_TemporaryEquityRedemptionOfPreferredStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_9cda9a7f-1c82-450b-a221-8778a5b4051f" xlink:to="loc_cmcsa_TemporaryEquityRedemptionOfPreferredStock_42232d11-cc85-40ae-9a35-cacff503879f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ContributionsFromAndDistributionsToNoncontrollingInterestsAttributableToTemporaryEquity_7c6be70c-b848-4a12-951b-f5e8b1ce4cb6" xlink:href="cmcsa-20210930.xsd#cmcsa_ContributionsFromAndDistributionsToNoncontrollingInterestsAttributableToTemporaryEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_9cda9a7f-1c82-450b-a221-8778a5b4051f" xlink:to="loc_cmcsa_ContributionsFromAndDistributionsToNoncontrollingInterestsAttributableToTemporaryEquity_7c6be70c-b848-4a12-951b-f5e8b1ce4cb6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityOtherChanges_f43e3148-96e0-40fd-bc94-5b00cebba062" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityOtherChanges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_9cda9a7f-1c82-450b-a221-8778a5b4051f" xlink:to="loc_us-gaap_TemporaryEquityOtherChanges_f43e3148-96e0-40fd-bc94-5b00cebba062" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest_82ed6eab-a4f1-4aa6-bdbf-b1c70ac8a5d8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_9cda9a7f-1c82-450b-a221-8778a5b4051f" xlink:to="loc_us-gaap_NetIncomeLossAttributableToRedeemableNoncontrollingInterest_82ed6eab-a4f1-4aa6-bdbf-b1c70ac8a5d8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount_e3b25ed5-3dd2-454e-a39a-2e48244b9601" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a31cb41a-3a24-45d0-8db7-0a63d68c9f79" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_32a780f1-5a9d-4dad-8b97-f03b5b42c09e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_32a780f1-5a9d-4dad-8b97-f03b5b42c09e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_5e0eace7-52b8-477a-b78d-db295bc908a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_5e0eace7-52b8-477a-b78d-db295bc908a3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueIssuedForServices_04c242a7-f8a2-43f9-8811-ace0f241a4cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueIssuedForServices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueIssuedForServices_04c242a7-f8a2-43f9-8811-ace0f241a4cd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_21cf919e-d63c-4c2a-8e4d-2890b73c548b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_21cf919e-d63c-4c2a-8e4d-2890b73c548b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_6f000a11-5dab-4bb5-803b-5876ca058f31" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_6f000a11-5dab-4bb5-803b-5876ca058f31" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStockCash_1cbf01e9-cd37-4d77-b1fe-a6b9df19e0aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DividendsCommonStockCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_DividendsCommonStockCash_1cbf01e9-cd37-4d77-b1fe-a6b9df19e0aa" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_1b044634-0d73-40c5-b394-ee36c89f3802" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_1b044634-0d73-40c5-b394-ee36c89f3802" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ContributionsFromAndDistributionsToNoncontrollingInterests_4fdeb920-fe7a-480b-95ef-9ce4fb9efa72" xlink:href="cmcsa-20210930.xsd#cmcsa_ContributionsFromAndDistributionsToNoncontrollingInterests"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_cmcsa_ContributionsFromAndDistributionsToNoncontrollingInterests_4fdeb920-fe7a-480b-95ef-9ce4fb9efa72" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityOther_6fd6356e-eeb6-46f2-aa41-20961e9f9f0f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_StockholdersEquityOther_6fd6356e-eeb6-46f2-aa41-20961e9f9f0f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_9887ca96-4d8e-47a7-8f7e-14ae95c54d56" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_NetIncomeLoss_9887ca96-4d8e-47a7-8f7e-14ae95c54d56" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_9da9277b-f262-4cd8-a092-dc71e648cadd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e3469048-8cb7-42cc-a3c7-b124d866add7" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_9da9277b-f262-4cd8-a092-dc71e648cadd" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_e0425b1f-d520-4cd4-8521-d718cf5da233" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockDividendsPerShareDeclared_cf67ff57-6f80-4753-811f-7298ae623962" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockDividendsPerShareDeclared"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a31cb41a-3a24-45d0-8db7-0a63d68c9f79" xlink:to="loc_us-gaap_CommonStockDividendsPerShareDeclared_cf67ff57-6f80-4753-811f-7298ae623962" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_7db67847-fd99-4890-bc30-87e09aba78cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_a31cb41a-3a24-45d0-8db7-0a63d68c9f79" xlink:to="loc_us-gaap_StatementTable_7db67847-fd99-4890-bc30-87e09aba78cd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_35dc80d2-1266-4390-baa6-95dc956fca4c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_7db67847-fd99-4890-bc30-87e09aba78cd" xlink:to="loc_us-gaap_StatementClassOfStockAxis_35dc80d2-1266-4390-baa6-95dc956fca4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_35dc80d2-1266-4390-baa6-95dc956fca4c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_35dc80d2-1266-4390-baa6-95dc956fca4c" xlink:to="loc_us-gaap_ClassOfStockDomain_35dc80d2-1266-4390-baa6-95dc956fca4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_cbaffd31-bdd0-4a90-aa23-bfc9410ec226" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_35dc80d2-1266-4390-baa6-95dc956fca4c" xlink:to="loc_us-gaap_ClassOfStockDomain_cbaffd31-bdd0-4a90-aa23-bfc9410ec226" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_81e8bb0c-8638-42a7-bad6-4a541880ffb0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_cbaffd31-bdd0-4a90-aa23-bfc9410ec226" xlink:to="loc_us-gaap_CommonClassAMember_81e8bb0c-8638-42a7-bad6-4a541880ffb0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_1e181cf8-6d01-4bd4-9daa-f4919c4dff5b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_7db67847-fd99-4890-bc30-87e09aba78cd" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_1e181cf8-6d01-4bd4-9daa-f4919c4dff5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_1e181cf8-6d01-4bd4-9daa-f4919c4dff5b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_1e181cf8-6d01-4bd4-9daa-f4919c4dff5b" xlink:to="loc_us-gaap_EquityComponentDomain_1e181cf8-6d01-4bd4-9daa-f4919c4dff5b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_1e181cf8-6d01-4bd4-9daa-f4919c4dff5b" xlink:to="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_RedeemableNoncontrollingInterestsAndRedeemableSubsidiaryPreferredStockMember_f4a3f470-ab34-4c1e-b068-0002a4b15f65" xlink:href="cmcsa-20210930.xsd#cmcsa_RedeemableNoncontrollingInterestsAndRedeemableSubsidiaryPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:to="loc_cmcsa_RedeemableNoncontrollingInterestsAndRedeemableSubsidiaryPreferredStockMember_f4a3f470-ab34-4c1e-b068-0002a4b15f65" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_9f9039b6-cd45-491e-a6fb-c42063b7e8aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:to="loc_us-gaap_CommonStockMember_9f9039b6-cd45-491e-a6fb-c42063b7e8aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_3b0ae9ac-53f6-49a0-999d-5afef0b5c117" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_3b0ae9ac-53f6-49a0-999d-5afef0b5c117" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_a965d914-fbb7-4e0f-95c9-3b8eaac4c7f9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:to="loc_us-gaap_RetainedEarningsMember_a965d914-fbb7-4e0f-95c9-3b8eaac4c7f9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockMember_0cf0d10b-b2c1-4722-acbc-c8f6c5029b20" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TreasuryStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:to="loc_us-gaap_TreasuryStockMember_0cf0d10b-b2c1-4722-acbc-c8f6c5029b20" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_9954e56d-ed36-4789-8420-e64471f3439f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_9954e56d-ed36-4789-8420-e64471f3439f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_d9816d3f-db1a-49f4-baff-f65b4e5df655" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_149a36fd-bdff-480e-a9d7-36255e2d572d" xlink:to="loc_us-gaap_NoncontrollingInterestMember_d9816d3f-db1a-49f4-baff-f65b4e5df655" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_9aa41b60-5a62-482d-ad55-89df9c6d4fd5" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_7db67847-fd99-4890-bc30-87e09aba78cd" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_9aa41b60-5a62-482d-ad55-89df9c6d4fd5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_9aa41b60-5a62-482d-ad55-89df9c6d4fd5_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_9aa41b60-5a62-482d-ad55-89df9c6d4fd5" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_9aa41b60-5a62-482d-ad55-89df9c6d4fd5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_5b392ec2-f39c-41a5-9e4c-0cc39aae4908" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_9aa41b60-5a62-482d-ad55-89df9c6d4fd5" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_5b392ec2-f39c-41a5-9e4c-0cc39aae4908" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_58eed262-bf95-4376-b5c4-2bdd60049654" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_5b392ec2-f39c-41a5-9e4c-0cc39aae4908" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_58eed262-bf95-4376-b5c4-2bdd60049654" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/SegmentInformationDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#SegmentInformationDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/SegmentInformationDetails" xlink:type="extended" id="i59f0e344d9a545adb8a545b4a14eaaa3_SegmentInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_4cc73a92-a445-4bcb-ae88-7892cf3b27a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_e6e9a3e4-1543-4804-930c-325f7f41463d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_4cc73a92-a445-4bcb-ae88-7892cf3b27a9" xlink:to="loc_us-gaap_NumberOfReportableSegments_e6e9a3e4-1543-4804-930c-325f7f41463d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationProfitLossAbstract_2a5920b9-5371-4efe-b4c0-7fff04bf82cc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationProfitLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_4cc73a92-a445-4bcb-ae88-7892cf3b27a9" xlink:to="loc_us-gaap_SegmentReportingInformationProfitLossAbstract_2a5920b9-5371-4efe-b4c0-7fff04bf82cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_80ebebc6-bc43-4a94-8154-d44401c10838" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationProfitLossAbstract_2a5920b9-5371-4efe-b4c0-7fff04bf82cc" xlink:to="loc_us-gaap_Revenues_80ebebc6-bc43-4a94-8154-d44401c10838" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_AdjustedEBITDA_a8f15e05-aa7b-415a-a1b0-ed0e57342c4e" xlink:href="cmcsa-20210930.xsd#cmcsa_AdjustedEBITDA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationProfitLossAbstract_2a5920b9-5371-4efe-b4c0-7fff04bf82cc" xlink:to="loc_cmcsa_AdjustedEBITDA_a8f15e05-aa7b-415a-a1b0-ed0e57342c4e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_9b53ce86-b0b9-447b-b564-0a9b045767dc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationProfitLossAbstract_2a5920b9-5371-4efe-b4c0-7fff04bf82cc" xlink:to="loc_us-gaap_DepreciationAndAmortization_9b53ce86-b0b9-447b-b564-0a9b045767dc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_ed798511-effc-4972-84ed-38ae67a334e4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationProfitLossAbstract_2a5920b9-5371-4efe-b4c0-7fff04bf82cc" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_ed798511-effc-4972-84ed-38ae67a334e4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireIntangibleAssets_3b794ac3-a6f2-4d6a-b8ef-b42ae065ae1d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationProfitLossAbstract_2a5920b9-5371-4efe-b4c0-7fff04bf82cc" xlink:to="loc_us-gaap_PaymentsToAcquireIntangibleAssets_3b794ac3-a6f2-4d6a-b8ef-b42ae065ae1d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ebb455e9-5232-43bb-bdbf-13748d1cfa12" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_4cc73a92-a445-4bcb-ae88-7892cf3b27a9" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ebb455e9-5232-43bb-bdbf-13748d1cfa12" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_b6c4eaa8-7f38-4c23-95a3-bb197fcbda1c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ebb455e9-5232-43bb-bdbf-13748d1cfa12" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_b6c4eaa8-7f38-4c23-95a3-bb197fcbda1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_b6c4eaa8-7f38-4c23-95a3-bb197fcbda1c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_b6c4eaa8-7f38-4c23-95a3-bb197fcbda1c" xlink:to="loc_us-gaap_SegmentDomain_b6c4eaa8-7f38-4c23-95a3-bb197fcbda1c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_b6c4eaa8-7f38-4c23-95a3-bb197fcbda1c" xlink:to="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CableCommunicationsMember_6b2be7f3-47ad-4d88-b5aa-81a3dd93e99b" xlink:href="cmcsa-20210930.xsd#cmcsa_CableCommunicationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:to="loc_cmcsa_CableCommunicationsMember_6b2be7f3-47ad-4d88-b5aa-81a3dd93e99b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_MediaMember_a32ebdd6-1ff9-400f-9124-8509ae35e75a" xlink:href="cmcsa-20210930.xsd#cmcsa_MediaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:to="loc_cmcsa_MediaMember_a32ebdd6-1ff9-400f-9124-8509ae35e75a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_StudiosMember_7110c1bf-3212-45b3-babb-dadf794501f9" xlink:href="cmcsa-20210930.xsd#cmcsa_StudiosMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:to="loc_cmcsa_StudiosMember_7110c1bf-3212-45b3-babb-dadf794501f9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ThemeParksMember_a610858a-6927-42cf-af70-8adc95839fe9" xlink:href="cmcsa-20210930.xsd#cmcsa_ThemeParksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:to="loc_cmcsa_ThemeParksMember_a610858a-6927-42cf-af70-8adc95839fe9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_HeadquartersAndOtherMember_a04e48ce-ad1a-4cdd-9a68-ea0130ece3cc" xlink:href="cmcsa-20210930.xsd#cmcsa_HeadquartersAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:to="loc_cmcsa_HeadquartersAndOtherMember_a04e48ce-ad1a-4cdd-9a68-ea0130ece3cc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_NBCUniversalSegmentsMember_e89422a3-a282-47b5-9b0f-de4c218c57e5" xlink:href="cmcsa-20210930.xsd#cmcsa_NBCUniversalSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:to="loc_cmcsa_NBCUniversalSegmentsMember_e89422a3-a282-47b5-9b0f-de4c218c57e5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_SkyLimitedMember_38192d00-103b-45de-90a3-ebbbffb7a926" xlink:href="cmcsa-20210930.xsd#cmcsa_SkyLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:to="loc_cmcsa_SkyLimitedMember_38192d00-103b-45de-90a3-ebbbffb7a926" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateAndOtherMember_4d486531-ec21-4339-bfdb-e1233991ec11" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_162b6eab-aa74-41ff-a358-c2adbf685f14" xlink:to="loc_us-gaap_CorporateAndOtherMember_4d486531-ec21-4339-bfdb-e1233991ec11" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_1c4942b0-c78c-4759-9094-cf32143aa3eb" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_ebb455e9-5232-43bb-bdbf-13748d1cfa12" xlink:to="loc_srt_ConsolidationItemsAxis_1c4942b0-c78c-4759-9094-cf32143aa3eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_1c4942b0-c78c-4759-9094-cf32143aa3eb_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_1c4942b0-c78c-4759-9094-cf32143aa3eb" xlink:to="loc_srt_ConsolidationItemsDomain_1c4942b0-c78c-4759-9094-cf32143aa3eb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_7d3c82b8-a16e-4ffa-b254-c4d2c8e2b596" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_1c4942b0-c78c-4759-9094-cf32143aa3eb" xlink:to="loc_srt_ConsolidationItemsDomain_7d3c82b8-a16e-4ffa-b254-c4d2c8e2b596" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_191d8d9c-a603-4160-9e51-55052baa31a0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_7d3c82b8-a16e-4ffa-b254-c4d2c8e2b596" xlink:to="loc_us-gaap_OperatingSegmentsMember_191d8d9c-a603-4160-9e51-55052baa31a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaterialReconcilingItemsMember_eeca4937-a2d4-4cfc-aafa-ba3daf38bfdc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MaterialReconcilingItemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_7d3c82b8-a16e-4ffa-b254-c4d2c8e2b596" xlink:to="loc_us-gaap_MaterialReconcilingItemsMember_eeca4937-a2d4-4cfc-aafa-ba3daf38bfdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_TotalIntersegmentEliminationsMember_0114fced-5649-46e4-8f89-de4ae53e31c5" xlink:href="cmcsa-20210930.xsd#cmcsa_TotalIntersegmentEliminationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaterialReconcilingItemsMember_eeca4937-a2d4-4cfc-aafa-ba3daf38bfdc" xlink:to="loc_cmcsa_TotalIntersegmentEliminationsMember_0114fced-5649-46e4-8f89-de4ae53e31c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_af3f98ab-59e4-41c0-bdb4-0c497b673cf5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_TotalIntersegmentEliminationsMember_0114fced-5649-46e4-8f89-de4ae53e31c5" xlink:to="loc_us-gaap_IntersegmentEliminationMember_af3f98ab-59e4-41c0-bdb4-0c497b673cf5" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/SegmentInformationReconciliationofAdjustedEBITDAfromSegmenttoConsolidatedStatementsDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#SegmentInformationReconciliationofAdjustedEBITDAfromSegmenttoConsolidatedStatementsDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/SegmentInformationReconciliationofAdjustedEBITDAfromSegmenttoConsolidatedStatementsDetails" xlink:type="extended" id="if0fb11b8df3a4140921a26a071e265c5_SegmentInformationReconciliationofAdjustedEBITDAfromSegmenttoConsolidatedStatementsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_AdjustedEBITDA_3f105595-aaf3-4767-9681-5fe179cf8ded" xlink:href="cmcsa-20210930.xsd#cmcsa_AdjustedEBITDA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:to="loc_cmcsa_AdjustedEBITDA_3f105595-aaf3-4767-9681-5fe179cf8ded" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_AdjustmentsForItemsExcludedFromAdjustedEBITDA_4a8a3d61-2350-400e-b99b-501aab86468b" xlink:href="cmcsa-20210930.xsd#cmcsa_AdjustmentsForItemsExcludedFromAdjustedEBITDA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:to="loc_cmcsa_AdjustmentsForItemsExcludedFromAdjustedEBITDA_4a8a3d61-2350-400e-b99b-501aab86468b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_018dddc4-024c-453a-870c-a167484f30d4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:to="loc_us-gaap_Depreciation_018dddc4-024c-453a-870c-a167484f30d4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_bf8d5fe2-9cf7-4f5a-854c-d460d3bbab77" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_bf8d5fe2-9cf7-4f5a-854c-d460d3bbab77" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_6d3b5710-8a2a-4727-9a28-20435bdd903c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:to="loc_us-gaap_InterestExpense_6d3b5710-8a2a-4727-9a28-20435bdd903c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_67cf59ba-dbd9-4640-8cfb-77637fe977c4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_67cf59ba-dbd9-4640-8cfb-77637fe977c4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_f7fe939f-4803-4166-bf8b-e11422678c9f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_f7fe939f-4803-4166-bf8b-e11422678c9f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_db6f0b31-f53e-4fa7-a9b1-6a133fe4e719" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_6cb50aaf-018c-4f5a-bce6-a7f05d1eedff" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_db6f0b31-f53e-4fa7-a9b1-6a133fe4e719" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_7796cae4-4e96-4f44-ad16-4125a9cde6a2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_db6f0b31-f53e-4fa7-a9b1-6a133fe4e719" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_7796cae4-4e96-4f44-ad16-4125a9cde6a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_7796cae4-4e96-4f44-ad16-4125a9cde6a2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_7796cae4-4e96-4f44-ad16-4125a9cde6a2" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_7796cae4-4e96-4f44-ad16-4125a9cde6a2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_f5ffca4f-7ca0-41cd-8bc9-e253365f8e38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_7796cae4-4e96-4f44-ad16-4125a9cde6a2" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_f5ffca4f-7ca0-41cd-8bc9-e253365f8e38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_GainLossRelatedToLegalSettlementMember_17842219-eea3-4b69-a71e-8ea0c1fd2105" xlink:href="cmcsa-20210930.xsd#cmcsa_GainLossRelatedToLegalSettlementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_f5ffca4f-7ca0-41cd-8bc9-e253365f8e38" xlink:to="loc_cmcsa_GainLossRelatedToLegalSettlementMember_17842219-eea3-4b69-a71e-8ea0c1fd2105" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/SegmentInformationScheduleofChangeinGoodwillDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#SegmentInformationScheduleofChangeinGoodwillDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/SegmentInformationScheduleofChangeinGoodwillDetails" xlink:type="extended" id="i45bddfde85334dd9ad8240d56a2754ae_SegmentInformationScheduleofChangeinGoodwillDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_69bb3a97-2648-43de-a1ff-35a8ad6333ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_3b27b715-0aab-4e92-8976-d72b2603e223" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_69bb3a97-2648-43de-a1ff-35a8ad6333ee" xlink:to="loc_us-gaap_GoodwillRollForward_3b27b715-0aab-4e92-8976-d72b2603e223" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_49239807-cba4-4b95-98b7-c7a507ce7d3d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_3b27b715-0aab-4e92-8976-d72b2603e223" xlink:to="loc_us-gaap_Goodwill_49239807-cba4-4b95-98b7-c7a507ce7d3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_GoodwillForeignCurrencyTranslationAndOther_ecf7764d-270d-460d-86f6-036e46c011bc" xlink:href="cmcsa-20210930.xsd#cmcsa_GoodwillForeignCurrencyTranslationAndOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_3b27b715-0aab-4e92-8976-d72b2603e223" xlink:to="loc_cmcsa_GoodwillForeignCurrencyTranslationAndOther_ecf7764d-270d-460d-86f6-036e46c011bc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_ffa3f0b1-fa17-4a68-849f-a9356b915955" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_031745ff-de5e-4d5f-8112-be82ca7ea30a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_69bb3a97-2648-43de-a1ff-35a8ad6333ee" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_031745ff-de5e-4d5f-8112-be82ca7ea30a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_f0bc201d-9dc8-4fd7-af26-7d75b0c08da0" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_031745ff-de5e-4d5f-8112-be82ca7ea30a" xlink:to="loc_srt_ConsolidationItemsAxis_f0bc201d-9dc8-4fd7-af26-7d75b0c08da0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_f0bc201d-9dc8-4fd7-af26-7d75b0c08da0_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_f0bc201d-9dc8-4fd7-af26-7d75b0c08da0" xlink:to="loc_srt_ConsolidationItemsDomain_f0bc201d-9dc8-4fd7-af26-7d75b0c08da0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_1e1500db-1046-49f2-b9bb-37c0880a5b47" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_f0bc201d-9dc8-4fd7-af26-7d75b0c08da0" xlink:to="loc_srt_ConsolidationItemsDomain_1e1500db-1046-49f2-b9bb-37c0880a5b47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_adb7695c-9e01-45b2-b15e-ec13f8d8eb53" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_1e1500db-1046-49f2-b9bb-37c0880a5b47" xlink:to="loc_us-gaap_OperatingSegmentsMember_adb7695c-9e01-45b2-b15e-ec13f8d8eb53" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_9f0d64f4-8c0f-4ca1-9611-9d224a5e42a0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_031745ff-de5e-4d5f-8112-be82ca7ea30a" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_9f0d64f4-8c0f-4ca1-9611-9d224a5e42a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_9f0d64f4-8c0f-4ca1-9611-9d224a5e42a0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_9f0d64f4-8c0f-4ca1-9611-9d224a5e42a0" xlink:to="loc_us-gaap_SegmentDomain_9f0d64f4-8c0f-4ca1-9611-9d224a5e42a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_9f0d64f4-8c0f-4ca1-9611-9d224a5e42a0" xlink:to="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CableCommunicationsMember_daf818c7-0ee2-45ae-9656-0f9c679df3c1" xlink:href="cmcsa-20210930.xsd#cmcsa_CableCommunicationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_cmcsa_CableCommunicationsMember_daf818c7-0ee2-45ae-9656-0f9c679df3c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CableNetworksMember_e8ed6c12-5baf-47e0-94bf-82cbe8956a3a" xlink:href="cmcsa-20210930.xsd#cmcsa_CableNetworksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_cmcsa_CableNetworksMember_e8ed6c12-5baf-47e0-94bf-82cbe8956a3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_BroadcastTelevisionMember_5343e154-ad62-47bd-a704-a106ce84281b" xlink:href="cmcsa-20210930.xsd#cmcsa_BroadcastTelevisionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_cmcsa_BroadcastTelevisionMember_5343e154-ad62-47bd-a704-a106ce84281b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_FilmedEntertainmentMember_47677d30-c7ed-4eb7-ad41-f22664b20eb8" xlink:href="cmcsa-20210930.xsd#cmcsa_FilmedEntertainmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_cmcsa_FilmedEntertainmentMember_47677d30-c7ed-4eb7-ad41-f22664b20eb8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_MediaMember_225365fd-0b4a-4aef-8be4-50e9a42226cf" xlink:href="cmcsa-20210930.xsd#cmcsa_MediaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_cmcsa_MediaMember_225365fd-0b4a-4aef-8be4-50e9a42226cf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_StudiosMember_c9af7ae5-afd2-4164-987f-65f281d62496" xlink:href="cmcsa-20210930.xsd#cmcsa_StudiosMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_cmcsa_StudiosMember_c9af7ae5-afd2-4164-987f-65f281d62496" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ThemeParksMember_dc3fb97c-fd27-49d1-a57f-72275f1aca1d" xlink:href="cmcsa-20210930.xsd#cmcsa_ThemeParksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_cmcsa_ThemeParksMember_dc3fb97c-fd27-49d1-a57f-72275f1aca1d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_SkyLimitedMember_18f23f2e-b157-418b-947c-810d0e66e85b" xlink:href="cmcsa-20210930.xsd#cmcsa_SkyLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_cmcsa_SkyLimitedMember_18f23f2e-b157-418b-947c-810d0e66e85b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateAndOtherMember_38e80c86-f597-4628-bb84-51b765a5d854" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_63eb123a-139e-4a29-adc2-e422a24209cc" xlink:to="loc_us-gaap_CorporateAndOtherMember_38e80c86-f597-4628-bb84-51b765a5d854" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationTypeAxis_0695c493-5fa0-44aa-a616-140386f449e6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ReclassificationTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_031745ff-de5e-4d5f-8112-be82ca7ea30a" xlink:to="loc_us-gaap_ReclassificationTypeAxis_0695c493-5fa0-44aa-a616-140386f449e6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationTypeDomain_0695c493-5fa0-44aa-a616-140386f449e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ReclassificationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationTypeAxis_0695c493-5fa0-44aa-a616-140386f449e6" xlink:to="loc_us-gaap_ReclassificationTypeDomain_0695c493-5fa0-44aa-a616-140386f449e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationTypeDomain_04db2417-1998-49cf-8709-f712a53d97f2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ReclassificationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationTypeAxis_0695c493-5fa0-44aa-a616-140386f449e6" xlink:to="loc_us-gaap_ReclassificationTypeDomain_04db2417-1998-49cf-8709-f712a53d97f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ReclassificationDueToSegmentChangeMember_1ea894c4-d9a5-47e0-bdda-f78e2c553e2f" xlink:href="cmcsa-20210930.xsd#cmcsa_ReclassificationDueToSegmentChangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationTypeDomain_04db2417-1998-49cf-8709-f712a53d97f2" xlink:to="loc_cmcsa_ReclassificationDueToSegmentChangeMember_1ea894c4-d9a5-47e0-bdda-f78e2c553e2f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/RevenueDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#RevenueDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/RevenueDetails" xlink:type="extended" id="i17e9d51102074420b760f2c28b265f11_RevenueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_5184e1ea-1752-4561-b273-b40177e4f57c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_22187620-c310-4706-b87f-875c55dd6ea7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_5184e1ea-1752-4561-b273-b40177e4f57c" xlink:to="loc_us-gaap_Revenues_22187620-c310-4706-b87f-875c55dd6ea7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_8d537e46-5861-4e7d-abb6-c88de7844378" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_5184e1ea-1752-4561-b273-b40177e4f57c" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_8d537e46-5861-4e7d-abb6-c88de7844378" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_3b47df3c-4e54-49b3-9739-b738445a32eb" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_8d537e46-5861-4e7d-abb6-c88de7844378" xlink:to="loc_srt_ConsolidationItemsAxis_3b47df3c-4e54-49b3-9739-b738445a32eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_3b47df3c-4e54-49b3-9739-b738445a32eb_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_3b47df3c-4e54-49b3-9739-b738445a32eb" xlink:to="loc_srt_ConsolidationItemsDomain_3b47df3c-4e54-49b3-9739-b738445a32eb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_6fa462ad-5dd6-4fe0-a7a8-6cfefe1f53c8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_3b47df3c-4e54-49b3-9739-b738445a32eb" xlink:to="loc_srt_ConsolidationItemsDomain_6fa462ad-5dd6-4fe0-a7a8-6cfefe1f53c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_73611bcf-5666-43a7-9b8f-92b493209549" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_6fa462ad-5dd6-4fe0-a7a8-6cfefe1f53c8" xlink:to="loc_us-gaap_OperatingSegmentsMember_73611bcf-5666-43a7-9b8f-92b493209549" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntersegmentEliminationMember_c5bb8886-d726-4eae-944e-decaba97a008" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IntersegmentEliminationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_6fa462ad-5dd6-4fe0-a7a8-6cfefe1f53c8" xlink:to="loc_us-gaap_IntersegmentEliminationMember_c5bb8886-d726-4eae-944e-decaba97a008" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_dffd8b75-3135-4666-8fb6-11bcbdd5c2c1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_8d537e46-5861-4e7d-abb6-c88de7844378" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_dffd8b75-3135-4666-8fb6-11bcbdd5c2c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_dffd8b75-3135-4666-8fb6-11bcbdd5c2c1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_dffd8b75-3135-4666-8fb6-11bcbdd5c2c1" xlink:to="loc_us-gaap_SegmentDomain_dffd8b75-3135-4666-8fb6-11bcbdd5c2c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_cdef30d7-ef34-4e76-8a32-8abfb4278906" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_dffd8b75-3135-4666-8fb6-11bcbdd5c2c1" xlink:to="loc_us-gaap_SegmentDomain_cdef30d7-ef34-4e76-8a32-8abfb4278906" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CableCommunicationsMember_9e13a542-9e1a-4340-8e7e-dfe2bcf4c612" xlink:href="cmcsa-20210930.xsd#cmcsa_CableCommunicationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_cdef30d7-ef34-4e76-8a32-8abfb4278906" xlink:to="loc_cmcsa_CableCommunicationsMember_9e13a542-9e1a-4340-8e7e-dfe2bcf4c612" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_NBCUniversalSegmentsMember_779d1245-7148-4d31-a140-6f4bcd96cd79" xlink:href="cmcsa-20210930.xsd#cmcsa_NBCUniversalSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_cdef30d7-ef34-4e76-8a32-8abfb4278906" xlink:to="loc_cmcsa_NBCUniversalSegmentsMember_779d1245-7148-4d31-a140-6f4bcd96cd79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_MediaMember_10b61dbc-cbe8-4616-9477-73de59f29997" xlink:href="cmcsa-20210930.xsd#cmcsa_MediaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_NBCUniversalSegmentsMember_779d1245-7148-4d31-a140-6f4bcd96cd79" xlink:to="loc_cmcsa_MediaMember_10b61dbc-cbe8-4616-9477-73de59f29997" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_StudiosMember_8bbad361-6633-40dc-a9f8-c1d7e87fc64e" xlink:href="cmcsa-20210930.xsd#cmcsa_StudiosMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_NBCUniversalSegmentsMember_779d1245-7148-4d31-a140-6f4bcd96cd79" xlink:to="loc_cmcsa_StudiosMember_8bbad361-6633-40dc-a9f8-c1d7e87fc64e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ThemeParksMember_7286b108-da8f-4e4e-bfe7-eeefa8d934e8" xlink:href="cmcsa-20210930.xsd#cmcsa_ThemeParksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_NBCUniversalSegmentsMember_779d1245-7148-4d31-a140-6f4bcd96cd79" xlink:to="loc_cmcsa_ThemeParksMember_7286b108-da8f-4e4e-bfe7-eeefa8d934e8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_HeadquartersAndOtherMember_e21b8855-8275-42dd-a115-4efc3ae60e5e" xlink:href="cmcsa-20210930.xsd#cmcsa_HeadquartersAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_NBCUniversalSegmentsMember_779d1245-7148-4d31-a140-6f4bcd96cd79" xlink:to="loc_cmcsa_HeadquartersAndOtherMember_e21b8855-8275-42dd-a115-4efc3ae60e5e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_SkyLimitedMember_8a639b3e-1fa2-44a4-8455-60668c6c24d5" xlink:href="cmcsa-20210930.xsd#cmcsa_SkyLimitedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_cdef30d7-ef34-4e76-8a32-8abfb4278906" xlink:to="loc_cmcsa_SkyLimitedMember_8a639b3e-1fa2-44a4-8455-60668c6c24d5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateAndOtherMember_54339f28-7c74-4f68-8858-b537303d5769" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_cdef30d7-ef34-4e76-8a32-8abfb4278906" xlink:to="loc_us-gaap_CorporateAndOtherMember_54339f28-7c74-4f68-8858-b537303d5769" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_043a8ab0-43f5-41ef-89af-e5ea92add4f1" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_8d537e46-5861-4e7d-abb6-c88de7844378" xlink:to="loc_srt_ProductOrServiceAxis_043a8ab0-43f5-41ef-89af-e5ea92add4f1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_043a8ab0-43f5-41ef-89af-e5ea92add4f1_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_043a8ab0-43f5-41ef-89af-e5ea92add4f1" xlink:to="loc_srt_ProductsAndServicesDomain_043a8ab0-43f5-41ef-89af-e5ea92add4f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_043a8ab0-43f5-41ef-89af-e5ea92add4f1" xlink:to="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ResidentialBroadbandMember_0af4065b-ac62-472c-9c0d-49e3008d3689" xlink:href="cmcsa-20210930.xsd#cmcsa_ResidentialBroadbandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_ResidentialBroadbandMember_0af4065b-ac62-472c-9c0d-49e3008d3689" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ResidentialVideoMember_08c1c4b9-204c-4862-96a5-bfa9a0262256" xlink:href="cmcsa-20210930.xsd#cmcsa_ResidentialVideoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_ResidentialVideoMember_08c1c4b9-204c-4862-96a5-bfa9a0262256" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ResidentialVoiceMember_54a12cbc-4cfb-4127-b162-fe0981386392" xlink:href="cmcsa-20210930.xsd#cmcsa_ResidentialVoiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_ResidentialVoiceMember_54a12cbc-4cfb-4127-b162-fe0981386392" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ResidentialWirelessMember_59396d3f-3684-47a5-8bdc-e42a8b0e19c1" xlink:href="cmcsa-20210930.xsd#cmcsa_ResidentialWirelessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_ResidentialWirelessMember_59396d3f-3684-47a5-8bdc-e42a8b0e19c1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_BusinessServicesMember_d8e2695c-384c-4c7c-9d7f-ca1662b9e8c7" xlink:href="cmcsa-20210930.xsd#cmcsa_BusinessServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_BusinessServicesMember_d8e2695c-384c-4c7c-9d7f-ca1662b9e8c7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_DistributionMember_ba115e86-846f-4197-b21e-33f0d1f25f34" xlink:href="cmcsa-20210930.xsd#cmcsa_DistributionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_DistributionMember_ba115e86-846f-4197-b21e-33f0d1f25f34" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ContentLicensingMember_caebd580-2e42-4302-8970-68badf68a3a0" xlink:href="cmcsa-20210930.xsd#cmcsa_ContentLicensingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_ContentLicensingMember_caebd580-2e42-4302-8970-68badf68a3a0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_DirectToConsumerMember_106d4811-25b8-41a6-86c4-542e2645b5c9" xlink:href="cmcsa-20210930.xsd#cmcsa_DirectToConsumerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_DirectToConsumerMember_106d4811-25b8-41a6-86c4-542e2645b5c9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ContentMember_2b9bf5c3-5979-4765-8f6f-e9b4399407b3" xlink:href="cmcsa-20210930.xsd#cmcsa_ContentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_ContentMember_2b9bf5c3-5979-4765-8f6f-e9b4399407b3" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdvertisingMember_86cfaa46-80bf-4474-b422-f3ff93583840" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdvertisingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_us-gaap_AdvertisingMember_86cfaa46-80bf-4474-b422-f3ff93583840" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_TheatricalMember_c8891a32-ad4d-4ebe-aa34-9acedbf8d363" xlink:href="cmcsa-20210930.xsd#cmcsa_TheatricalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_TheatricalMember_c8891a32-ad4d-4ebe-aa34-9acedbf8d363" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_HomeEntertainmentAndOtherMember_2a96b948-8626-4a56-9e71-55f44fe2f926" xlink:href="cmcsa-20210930.xsd#cmcsa_HomeEntertainmentAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_HomeEntertainmentAndOtherMember_2a96b948-8626-4a56-9e71-55f44fe2f926" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_OtherMember_a6107e8b-f44b-4e42-9f5f-13a168fe0fb0" xlink:href="cmcsa-20210930.xsd#cmcsa_OtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_d073dd49-bed2-4ecd-9b62-0aa3c8592a47" xlink:to="loc_cmcsa_OtherMember_a6107e8b-f44b-4e42-9f5f-13a168fe0fb0" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/RevenueRevenuebyGeographicLocationDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#RevenueRevenuebyGeographicLocationDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/RevenueRevenuebyGeographicLocationDetails" xlink:type="extended" id="ia2a437ffe4274a1eae1a78276235e628_RevenueRevenuebyGeographicLocationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_fdcdbd16-75f1-4fda-bf36-4733d28eb545" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_29f4154f-1172-43eb-ae4f-7bdcffcfbd80" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_fdcdbd16-75f1-4fda-bf36-4733d28eb545" xlink:to="loc_us-gaap_Revenues_29f4154f-1172-43eb-ae4f-7bdcffcfbd80" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_9bf47f49-7343-4770-aa34-b1eacda99315" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_fdcdbd16-75f1-4fda-bf36-4733d28eb545" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_9bf47f49-7343-4770-aa34-b1eacda99315" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_1cf7897f-6991-4255-a260-38c99130aadc" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_9bf47f49-7343-4770-aa34-b1eacda99315" xlink:to="loc_srt_StatementGeographicalAxis_1cf7897f-6991-4255-a260-38c99130aadc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_1cf7897f-6991-4255-a260-38c99130aadc_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_1cf7897f-6991-4255-a260-38c99130aadc" xlink:to="loc_srt_SegmentGeographicalDomain_1cf7897f-6991-4255-a260-38c99130aadc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_4896437a-c9e6-4d7e-9519-d2ed40b5b9ba" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_1cf7897f-6991-4255-a260-38c99130aadc" xlink:to="loc_srt_SegmentGeographicalDomain_4896437a-c9e6-4d7e-9519-d2ed40b5b9ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_d3642f36-53de-4791-a576-9763e4050594" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4896437a-c9e6-4d7e-9519-d2ed40b5b9ba" xlink:to="loc_country_US_d3642f36-53de-4791-a576-9763e4050594" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EuropeMember_24c0f06d-4289-4e0e-974e-c21a322cbda6" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EuropeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4896437a-c9e6-4d7e-9519-d2ed40b5b9ba" xlink:to="loc_srt_EuropeMember_24c0f06d-4289-4e0e-974e-c21a322cbda6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_OtherGeographicLocationsMember_fdc7ba63-69a9-4080-8de5-16bb46eea9aa" xlink:href="cmcsa-20210930.xsd#cmcsa_OtherGeographicLocationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_4896437a-c9e6-4d7e-9519-d2ed40b5b9ba" xlink:to="loc_cmcsa_OtherGeographicLocationsMember_fdc7ba63-69a9-4080-8de5-16bb46eea9aa" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/LongTermDebtNarrativeDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#LongTermDebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/LongTermDebtNarrativeDetails" xlink:type="extended" id="i0129ff81f0024e9d93f30aee4866525f_LongTermDebtNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_ce458063-f79a-4da6-98c4-209e1d459655" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndCapitalLeaseObligations_89ef31d1-c7b3-4a4b-81d1-ef9bfa1769aa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ce458063-f79a-4da6-98c4-209e1d459655" xlink:to="loc_us-gaap_DebtAndCapitalLeaseObligations_89ef31d1-c7b3-4a4b-81d1-ef9bfa1769aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_d9cf4465-7fae-4d1a-8c63-cd4b796f8e62" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ce458063-f79a-4da6-98c4-209e1d459655" xlink:to="loc_us-gaap_LongTermDebtFairValue_d9cf4465-7fae-4d1a-8c63-cd4b796f8e62" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_19921279-e0e3-460b-b8eb-a10cfaa043d7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ce458063-f79a-4da6-98c4-209e1d459655" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_19921279-e0e3-460b-b8eb-a10cfaa043d7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_LineOfCreditFacilityPotentialIncreaseToMaximumBorrowingCapacity_8650efbb-7f12-4c79-b4b5-0a0f47558fcd" xlink:href="cmcsa-20210930.xsd#cmcsa_LineOfCreditFacilityPotentialIncreaseToMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ce458063-f79a-4da6-98c4-209e1d459655" xlink:to="loc_cmcsa_LineOfCreditFacilityPotentialIncreaseToMaximumBorrowingCapacity_8650efbb-7f12-4c79-b4b5-0a0f47558fcd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_c9875393-83ac-4791-949b-f7212a6e4559" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ce458063-f79a-4da6-98c4-209e1d459655" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_c9875393-83ac-4791-949b-f7212a6e4559" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_b3f64c37-27b1-4574-9a77-35d7e1fb690a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ce458063-f79a-4da6-98c4-209e1d459655" xlink:to="loc_us-gaap_LineOfCredit_b3f64c37-27b1-4574-9a77-35d7e1fb690a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_58185dd9-5c47-452c-a736-342ca10c54a2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ce458063-f79a-4da6-98c4-209e1d459655" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_58185dd9-5c47-452c-a736-342ca10c54a2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_d1506f2c-ee3a-421b-980d-a1b187687ce0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_58185dd9-5c47-452c-a736-342ca10c54a2" xlink:to="loc_us-gaap_CreditFacilityAxis_d1506f2c-ee3a-421b-980d-a1b187687ce0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_d1506f2c-ee3a-421b-980d-a1b187687ce0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_d1506f2c-ee3a-421b-980d-a1b187687ce0" xlink:to="loc_us-gaap_CreditFacilityDomain_d1506f2c-ee3a-421b-980d-a1b187687ce0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_9f735467-766a-42a7-ae28-dcee87227533" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_d1506f2c-ee3a-421b-980d-a1b187687ce0" xlink:to="loc_us-gaap_CreditFacilityDomain_9f735467-766a-42a7-ae28-dcee87227533" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_0a9dde8f-adba-47c8-ba37-35aba4578eca" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_9f735467-766a-42a7-ae28-dcee87227533" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_0a9dde8f-adba-47c8-ba37-35aba4578eca" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_f7c86416-7a06-4fca-9a29-90f018bcc341" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_58185dd9-5c47-452c-a736-342ca10c54a2" xlink:to="loc_us-gaap_VariableRateAxis_f7c86416-7a06-4fca-9a29-90f018bcc341" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_f7c86416-7a06-4fca-9a29-90f018bcc341_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_f7c86416-7a06-4fca-9a29-90f018bcc341" xlink:to="loc_us-gaap_VariableRateDomain_f7c86416-7a06-4fca-9a29-90f018bcc341_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_969771c4-fee9-4361-9923-08d3c36901ae" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_f7c86416-7a06-4fca-9a29-90f018bcc341" xlink:to="loc_us-gaap_VariableRateDomain_969771c4-fee9-4361-9923-08d3c36901ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_4a552313-b2f3-4536-a82e-5b26ec924dcc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_969771c4-fee9-4361-9923-08d3c36901ae" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_4a552313-b2f3-4536-a82e-5b26ec924dcc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_852a3801-8632-4bf4-b221-f6fc9e651a25" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_58185dd9-5c47-452c-a736-342ca10c54a2" xlink:to="loc_us-gaap_DebtInstrumentAxis_852a3801-8632-4bf4-b221-f6fc9e651a25" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_852a3801-8632-4bf4-b221-f6fc9e651a25_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_852a3801-8632-4bf4-b221-f6fc9e651a25" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_852a3801-8632-4bf4-b221-f6fc9e651a25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_9b99b021-5397-4abe-91ae-a642c0fc1108" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_852a3801-8632-4bf4-b221-f6fc9e651a25" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_9b99b021-5397-4abe-91ae-a642c0fc1108" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_4eb199cf-cda4-4365-9d71-435100c76945" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_9b99b021-5397-4abe-91ae-a642c0fc1108" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_4eb199cf-cda4-4365-9d71-435100c76945" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/SignificantTransactionsUniversalBeijingResortDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#SignificantTransactionsUniversalBeijingResortDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/SignificantTransactionsUniversalBeijingResortDetails" xlink:type="extended" id="i4eba741a4ecc46cd9feca19585bdf9ea_SignificantTransactionsUniversalBeijingResortDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityLineItems_2e691ba4-85d3-4506-ae78-7d96be73e374" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableInterestEntityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_d7412499-d517-42f9-98e0-4dea88c9aba4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_2e691ba4-85d3-4506-ae78-7d96be73e374" xlink:to="loc_us-gaap_MinorityInterestOwnershipPercentageByParent_d7412499-d517-42f9-98e0-4dea88c9aba4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_e3219067-b389-4dc4-9e93-0cee205aa6ec" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_2e691ba4-85d3-4506-ae78-7d96be73e374" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_e3219067-b389-4dc4-9e93-0cee205aa6ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLongTermDebt_be855903-189e-4bec-b3b4-233bb471223b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_2e691ba4-85d3-4506-ae78-7d96be73e374" xlink:to="loc_us-gaap_OtherLongTermDebt_be855903-189e-4bec-b3b4-233bb471223b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_f3fb1fa5-0468-46ed-8789-32dd0b36abb8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_2e691ba4-85d3-4506-ae78-7d96be73e374" xlink:to="loc_us-gaap_Assets_f3fb1fa5-0468-46ed-8789-32dd0b36abb8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_04a4b365-47f8-4cbb-95c2-11cd68a5a9fc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_2e691ba4-85d3-4506-ae78-7d96be73e374" xlink:to="loc_us-gaap_Liabilities_04a4b365-47f8-4cbb-95c2-11cd68a5a9fc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_2b61c311-4bdc-4f42-b1f0-5bbaa0a39442" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_2e691ba4-85d3-4506-ae78-7d96be73e374" xlink:to="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_2b61c311-4bdc-4f42-b1f0-5bbaa0a39442" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipAxis_75b5eb49-afb6-481e-9118-aa683e065f90" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_OwnershipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_2b61c311-4bdc-4f42-b1f0-5bbaa0a39442" xlink:to="loc_srt_OwnershipAxis_75b5eb49-afb6-481e-9118-aa683e065f90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_75b5eb49-afb6-481e-9118-aa683e065f90_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_OwnershipAxis_75b5eb49-afb6-481e-9118-aa683e065f90" xlink:to="loc_srt_OwnershipDomain_75b5eb49-afb6-481e-9118-aa683e065f90_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OwnershipDomain_cecb853a-3249-485a-a995-9b629c023c71" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_OwnershipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_OwnershipAxis_75b5eb49-afb6-481e-9118-aa683e065f90" xlink:to="loc_srt_OwnershipDomain_cecb853a-3249-485a-a995-9b629c023c71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_UniversalBeijingResortMember_1a83a002-7a34-42ff-921f-dc5ae48be904" xlink:href="cmcsa-20210930.xsd#cmcsa_UniversalBeijingResortMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OwnershipDomain_cecb853a-3249-485a-a995-9b629c023c71" xlink:to="loc_cmcsa_UniversalBeijingResortMember_1a83a002-7a34-42ff-921f-dc5ae48be904" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_9b287564-d557-4cf6-9594-8cfe82d1ca16" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_2b61c311-4bdc-4f42-b1f0-5bbaa0a39442" xlink:to="loc_srt_ConsolidatedEntitiesAxis_9b287564-d557-4cf6-9594-8cfe82d1ca16" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_9b287564-d557-4cf6-9594-8cfe82d1ca16_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_9b287564-d557-4cf6-9594-8cfe82d1ca16" xlink:to="loc_srt_ConsolidatedEntitiesDomain_9b287564-d557-4cf6-9594-8cfe82d1ca16_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_312e7a9d-c8be-4e94-bd90-fdac30c45a50" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_9b287564-d557-4cf6-9594-8cfe82d1ca16" xlink:to="loc_srt_ConsolidatedEntitiesDomain_312e7a9d-c8be-4e94-bd90-fdac30c45a50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_654601e6-ab83-4265-9e09-4b25cae079b7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableInterestEntityPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_312e7a9d-c8be-4e94-bd90-fdac30c45a50" xlink:to="loc_us-gaap_VariableInterestEntityPrimaryBeneficiaryMember_654601e6-ab83-4265-9e09-4b25cae079b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_a8f26a6a-48b3-450a-a9b2-1a56a05730b9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_2b61c311-4bdc-4f42-b1f0-5bbaa0a39442" xlink:to="loc_us-gaap_DebtInstrumentAxis_a8f26a6a-48b3-450a-a9b2-1a56a05730b9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_a8f26a6a-48b3-450a-a9b2-1a56a05730b9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_a8f26a6a-48b3-450a-a9b2-1a56a05730b9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_a8f26a6a-48b3-450a-a9b2-1a56a05730b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c5f58179-326a-4411-afbe-9f58afcc2c49" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_a8f26a6a-48b3-450a-a9b2-1a56a05730b9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c5f58179-326a-4411-afbe-9f58afcc2c49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_UniversalBeijingResortTermLoansMember_4184f3a9-d4b4-4867-ad21-a6da8249a070" xlink:href="cmcsa-20210930.xsd#cmcsa_UniversalBeijingResortTermLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c5f58179-326a-4411-afbe-9f58afcc2c49" xlink:to="loc_cmcsa_UniversalBeijingResortTermLoansMember_4184f3a9-d4b4-4867-ad21-a6da8249a070" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/SignificantTransactionsAcquisitionsDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#SignificantTransactionsAcquisitionsDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/SignificantTransactionsAcquisitionsDetails" xlink:type="extended" id="i7742cd4c01c64fa88727fff457848a31_SignificantTransactionsAcquisitionsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsLineItems_5f2c1ef5-b42c-4400-adee-abb1be2159d5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_46d2ddd8-53af-4f1f-8abc-3a3c9a84acff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsLineItems_5f2c1ef5-b42c-4400-adee-abb1be2159d5" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_46d2ddd8-53af-4f1f-8abc-3a3c9a84acff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsTable_cd04c4de-2aab-4dd5-bff4-a9ddcea37a6b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsLineItems_5f2c1ef5-b42c-4400-adee-abb1be2159d5" xlink:to="loc_us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsTable_cd04c4de-2aab-4dd5-bff4-a9ddcea37a6b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_c870bf36-ea0c-4e9d-83c5-78f95f44e5fb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsTable_cd04c4de-2aab-4dd5-bff4-a9ddcea37a6b" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_c870bf36-ea0c-4e9d-83c5-78f95f44e5fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c870bf36-ea0c-4e9d-83c5-78f95f44e5fb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_c870bf36-ea0c-4e9d-83c5-78f95f44e5fb" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_c870bf36-ea0c-4e9d-83c5-78f95f44e5fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d4de8a8b-388c-4b44-9897-33738701b593" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_c870bf36-ea0c-4e9d-83c5-78f95f44e5fb" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d4de8a8b-388c-4b44-9897-33738701b593" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_MasergyMember_30b6fe72-bcd9-42dd-b9cf-ddf66476f7e3" xlink:href="cmcsa-20210930.xsd#cmcsa_MasergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_d4de8a8b-388c-4b44-9897-33738701b593" xlink:to="loc_cmcsa_MasergyMember_30b6fe72-bcd9-42dd-b9cf-ddf66476f7e3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_c0968e51-6895-4058-8b27-01a284231e84" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsTable_cd04c4de-2aab-4dd5-bff4-a9ddcea37a6b" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_c0968e51-6895-4058-8b27-01a284231e84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_c0968e51-6895-4058-8b27-01a284231e84_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_c0968e51-6895-4058-8b27-01a284231e84" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_c0968e51-6895-4058-8b27-01a284231e84_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_6b253fd8-e87a-4770-bd63-4c026c084f7a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_c0968e51-6895-4058-8b27-01a284231e84" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_6b253fd8-e87a-4770-bd63-4c026c084f7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_1555034b-15e0-45db-8f80-58f8c5d0a5d3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_6b253fd8-e87a-4770-bd63-4c026c084f7a" xlink:to="loc_us-gaap_SubsequentEventMember_1555034b-15e0-45db-8f80-58f8c5d0a5d3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/InvestmentsEquityMethodInvestmentsDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#InvestmentsEquityMethodInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/InvestmentsEquityMethodInvestmentsDetails" xlink:type="extended" id="i7ceb4d4b1e9c44fa810cc11490b50c89_InvestmentsEquityMethodInvestmentsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_e4a92dbb-6d36-4909-9af4-03b2659b5c97" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CapitalContributionToVIE_22f98377-c447-4ffa-8330-3f0e8b7fe637" xlink:href="cmcsa-20210930.xsd#cmcsa_CapitalContributionToVIE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_e4a92dbb-6d36-4909-9af4-03b2659b5c97" xlink:to="loc_cmcsa_CapitalContributionToVIE_22f98377-c447-4ffa-8330-3f0e8b7fe637" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_EquityMethodInvestmentsAndCertainRetainedDistributions_72d689c6-d7b6-4f88-9cab-921e631d5a02" xlink:href="cmcsa-20210930.xsd#cmcsa_EquityMethodInvestmentsAndCertainRetainedDistributions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_e4a92dbb-6d36-4909-9af4-03b2659b5c97" xlink:to="loc_cmcsa_EquityMethodInvestmentsAndCertainRetainedDistributions_72d689c6-d7b6-4f88-9cab-921e631d5a02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_VariableInterestEntityReportingEntityInvolvementUnfundedCapitalCommitment_ed139063-7dcd-46a7-903c-00bd699b8de1" xlink:href="cmcsa-20210930.xsd#cmcsa_VariableInterestEntityReportingEntityInvolvementUnfundedCapitalCommitment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_e4a92dbb-6d36-4909-9af4-03b2659b5c97" xlink:to="loc_cmcsa_VariableInterestEntityReportingEntityInvolvementUnfundedCapitalCommitment_ed139063-7dcd-46a7-903c-00bd699b8de1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ProceedsFromCollateralizedObligation_68d41b72-e68e-43e5-a93d-d1a65b1a24ce" xlink:href="cmcsa-20210930.xsd#cmcsa_ProceedsFromCollateralizedObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_e4a92dbb-6d36-4909-9af4-03b2659b5c97" xlink:to="loc_cmcsa_ProceedsFromCollateralizedObligation_68d41b72-e68e-43e5-a93d-d1a65b1a24ce" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CollateralizedObligation_074c2b3d-2265-4d6a-9aed-28cf00db73b3" xlink:href="cmcsa-20210930.xsd#cmcsa_CollateralizedObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_e4a92dbb-6d36-4909-9af4-03b2659b5c97" xlink:to="loc_cmcsa_CollateralizedObligation_074c2b3d-2265-4d6a-9aed-28cf00db73b3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CollateralizedObligationFairValue_ccdac2f9-6e51-42f6-bd20-12946a63f8bd" xlink:href="cmcsa-20210930.xsd#cmcsa_CollateralizedObligationFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_e4a92dbb-6d36-4909-9af4-03b2659b5c97" xlink:to="loc_cmcsa_CollateralizedObligationFairValue_ccdac2f9-6e51-42f6-bd20-12946a63f8bd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_553ba7f9-560b-42a2-a1e5-fd2f1d9270a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_e4a92dbb-6d36-4909-9af4-03b2659b5c97" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_553ba7f9-560b-42a2-a1e5-fd2f1d9270a3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_8f56a14e-aa1d-40e6-871e-1436f77b12d4" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_553ba7f9-560b-42a2-a1e5-fd2f1d9270a3" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_8f56a14e-aa1d-40e6-871e-1436f77b12d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_8f56a14e-aa1d-40e6-871e-1436f77b12d4_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_8f56a14e-aa1d-40e6-871e-1436f77b12d4" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_8f56a14e-aa1d-40e6-871e-1436f77b12d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_2bd661f9-1fa1-4660-8c97-53485598b85f" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_8f56a14e-aa1d-40e6-871e-1436f77b12d4" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_2bd661f9-1fa1-4660-8c97-53485598b85f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_AtairosMember_51f7a054-a24a-458e-928b-37321c30f654" xlink:href="cmcsa-20210930.xsd#cmcsa_AtairosMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_2bd661f9-1fa1-4660-8c97-53485598b85f" xlink:to="loc_cmcsa_AtairosMember_51f7a054-a24a-458e-928b-37321c30f654" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_HuluMember_92217055-5957-4686-bae8-9e5718fe10d4" xlink:href="cmcsa-20210930.xsd#cmcsa_HuluMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_2bd661f9-1fa1-4660-8c97-53485598b85f" xlink:to="loc_cmcsa_HuluMember_92217055-5957-4686-bae8-9e5718fe10d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesAxis_a3a65aaf-63c3-4d49-b6bc-e7b22422f3d1" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidatedEntitiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_553ba7f9-560b-42a2-a1e5-fd2f1d9270a3" xlink:to="loc_srt_ConsolidatedEntitiesAxis_a3a65aaf-63c3-4d49-b6bc-e7b22422f3d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_a3a65aaf-63c3-4d49-b6bc-e7b22422f3d1_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidatedEntitiesAxis_a3a65aaf-63c3-4d49-b6bc-e7b22422f3d1" xlink:to="loc_srt_ConsolidatedEntitiesDomain_a3a65aaf-63c3-4d49-b6bc-e7b22422f3d1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidatedEntitiesDomain_06a40163-bdf4-44b8-ad67-e06e02253363" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidatedEntitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidatedEntitiesAxis_a3a65aaf-63c3-4d49-b6bc-e7b22422f3d1" xlink:to="loc_srt_ConsolidatedEntitiesDomain_06a40163-bdf4-44b8-ad67-e06e02253363" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember_f7ec3da2-8e88-4a8a-bad1-09488683e3cb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidatedEntitiesDomain_06a40163-bdf4-44b8-ad67-e06e02253363" xlink:to="loc_us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember_f7ec3da2-8e88-4a8a-bad1-09488683e3cb" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/InvestmentsOtherInvestmentsDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#InvestmentsOtherInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/InvestmentsOtherInvestmentsDetails" xlink:type="extended" id="iffdbf384c48a428cb197dd827dd14fc5_InvestmentsOtherInvestmentsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestmentNotReadilyMarketableLineItems_1cbfee8b-10d5-4eaa-85e4-cd81532a0177" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherInvestmentNotReadilyMarketableLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities_09cea46b-516a-42c6-8bde-87ebd903732d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherInvestmentNotReadilyMarketableLineItems_1cbfee8b-10d5-4eaa-85e4-cd81532a0177" xlink:to="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfHeldToMaturitySecurities_09cea46b-516a-42c6-8bde-87ebd903732d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_NumberOfPreferredStockSeries_0a891fad-e373-46e2-8092-e2f45ecf0592" xlink:href="cmcsa-20210930.xsd#cmcsa_NumberOfPreferredStockSeries"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherInvestmentNotReadilyMarketableLineItems_1cbfee8b-10d5-4eaa-85e4-cd81532a0177" xlink:to="loc_cmcsa_NumberOfPreferredStockSeries_0a891fad-e373-46e2-8092-e2f45ecf0592" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_PaymentsForRepurchaseandRedemptionofSubsidiaryPreferredStock_058ce422-af11-4e51-b059-439b2a776017" xlink:href="cmcsa-20210930.xsd#cmcsa_PaymentsForRepurchaseandRedemptionofSubsidiaryPreferredStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherInvestmentNotReadilyMarketableLineItems_1cbfee8b-10d5-4eaa-85e4-cd81532a0177" xlink:to="loc_cmcsa_PaymentsForRepurchaseandRedemptionofSubsidiaryPreferredStock_058ce422-af11-4e51-b059-439b2a776017" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherInvestmentNotReadilyMarketableTable_8fb12478-1a54-4781-b183-37d75d867e41" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherInvestmentNotReadilyMarketableTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherInvestmentNotReadilyMarketableLineItems_1cbfee8b-10d5-4eaa-85e4-cd81532a0177" xlink:to="loc_us-gaap_OtherInvestmentNotReadilyMarketableTable_8fb12478-1a54-4781-b183-37d75d867e41" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_9b403afa-1efe-46ef-a8d9-d7089cdb9137" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherInvestmentNotReadilyMarketableTable_8fb12478-1a54-4781-b183-37d75d867e41" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_9b403afa-1efe-46ef-a8d9-d7089cdb9137" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_9b403afa-1efe-46ef-a8d9-d7089cdb9137_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_9b403afa-1efe-46ef-a8d9-d7089cdb9137" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_9b403afa-1efe-46ef-a8d9-d7089cdb9137_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_c7fc541b-5961-48ac-b712-b61135ec8417" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_9b403afa-1efe-46ef-a8d9-d7089cdb9137" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_c7fc541b-5961-48ac-b712-b61135ec8417" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_AirTouchMember_ef0b4843-5fb9-41e5-ac60-1f663e179f08" xlink:href="cmcsa-20210930.xsd#cmcsa_AirTouchMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_c7fc541b-5961-48ac-b712-b61135ec8417" xlink:to="loc_cmcsa_AirTouchMember_ef0b4843-5fb9-41e5-ac60-1f663e179f08" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/EquityandShareBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#EquityandShareBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/EquityandShareBasedCompensationNarrativeDetails" xlink:type="extended" id="i856d5551a02e43dd82b91f5ebf729a7a_EquityandShareBasedCompensationNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ec268aa9-efef-4c65-ab74-00fc3a0068e9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_aff7f08a-8ef5-4706-b6f6-276b64d2dbf7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ec268aa9-efef-4c65-ab74-00fc3a0068e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_aff7f08a-8ef5-4706-b6f6-276b64d2dbf7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_641558c8-f7ce-49e6-98fe-e2232c5a940d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ec268aa9-efef-4c65-ab74-00fc3a0068e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_641558c8-f7ce-49e6-98fe-e2232c5a940d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_4de18300-6b07-4a4b-8fc6-149e7d369309" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ec268aa9-efef-4c65-ab74-00fc3a0068e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_4de18300-6b07-4a4b-8fc6-149e7d369309" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_c284b570-5f43-45f2-b54c-897c0788efc8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ec268aa9-efef-4c65-ab74-00fc3a0068e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_c284b570-5f43-45f2-b54c-897c0788efc8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_53ac97dd-f19c-4641-acb6-ff6b43bb9ad1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ec268aa9-efef-4c65-ab74-00fc3a0068e9" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_53ac97dd-f19c-4641-acb6-ff6b43bb9ad1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3f1c1030-1579-4817-aa16-3118f07c2927" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_ec268aa9-efef-4c65-ab74-00fc3a0068e9" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3f1c1030-1579-4817-aa16-3118f07c2927" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_35d0a7c6-ee08-4dd0-9eef-328d37e6cb45" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3f1c1030-1579-4817-aa16-3118f07c2927" xlink:to="loc_us-gaap_AwardTypeAxis_35d0a7c6-ee08-4dd0-9eef-328d37e6cb45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_35d0a7c6-ee08-4dd0-9eef-328d37e6cb45_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_35d0a7c6-ee08-4dd0-9eef-328d37e6cb45" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_35d0a7c6-ee08-4dd0-9eef-328d37e6cb45_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_63cb05a8-4da3-4ae1-9f0c-9356ea7708ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_35d0a7c6-ee08-4dd0-9eef-328d37e6cb45" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_63cb05a8-4da3-4ae1-9f0c-9356ea7708ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_0f51ddda-9bc3-411b-8031-d62a44367434" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_63cb05a8-4da3-4ae1-9f0c-9356ea7708ff" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_0f51ddda-9bc3-411b-8031-d62a44367434" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_5dbc8f69-0125-4b98-bc89-c36d1b8a459d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_63cb05a8-4da3-4ae1-9f0c-9356ea7708ff" xlink:to="loc_us-gaap_EmployeeStockOptionMember_5dbc8f69-0125-4b98-bc89-c36d1b8a459d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_8f0b4ea1-ee24-4998-89c6-b0e043926b91" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3f1c1030-1579-4817-aa16-3118f07c2927" xlink:to="loc_us-gaap_PlanNameAxis_8f0b4ea1-ee24-4998-89c6-b0e043926b91" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_8f0b4ea1-ee24-4998-89c6-b0e043926b91_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_8f0b4ea1-ee24-4998-89c6-b0e043926b91" xlink:to="loc_us-gaap_PlanNameDomain_8f0b4ea1-ee24-4998-89c6-b0e043926b91_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_0cf929fd-e547-4e42-8457-e2bd3d33345e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_8f0b4ea1-ee24-4998-89c6-b0e043926b91" xlink:to="loc_us-gaap_PlanNameDomain_0cf929fd-e547-4e42-8457-e2bd3d33345e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ManagementGrantMember_aef3f574-7fe9-415e-948e-667ecc39d81d" xlink:href="cmcsa-20210930.xsd#cmcsa_ManagementGrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_0cf929fd-e547-4e42-8457-e2bd3d33345e" xlink:to="loc_cmcsa_ManagementGrantMember_aef3f574-7fe9-415e-948e-667ecc39d81d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/EquityandShareBasedCompensationRecognizedShareBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#EquityandShareBasedCompensationRecognizedShareBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/EquityandShareBasedCompensationRecognizedShareBasedCompensationExpenseDetails" xlink:type="extended" id="i11f15b2b0afb47db924ba3d3f1ebf87e_EquityandShareBasedCompensationRecognizedShareBasedCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_66b1282c-3ec0-4085-affc-26ea8907f4af" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_80e1701d-4440-42f0-84b0-00301fda6117" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_66b1282c-3ec0-4085-affc-26ea8907f4af" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_80e1701d-4440-42f0-84b0-00301fda6117" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c4be124-dd8d-4eb9-9b1a-fb16a8591b42" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_66b1282c-3ec0-4085-affc-26ea8907f4af" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c4be124-dd8d-4eb9-9b1a-fb16a8591b42" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_cd863643-3710-48bf-9942-edd1b24f7cb9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1c4be124-dd8d-4eb9-9b1a-fb16a8591b42" xlink:to="loc_us-gaap_AwardTypeAxis_cd863643-3710-48bf-9942-edd1b24f7cb9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cd863643-3710-48bf-9942-edd1b24f7cb9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_cd863643-3710-48bf-9942-edd1b24f7cb9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cd863643-3710-48bf-9942-edd1b24f7cb9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f313ba2f-1437-408e-9973-eeb11d6c56a0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_cd863643-3710-48bf-9942-edd1b24f7cb9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f313ba2f-1437-408e-9973-eeb11d6c56a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_2ea5a6e9-25c3-4069-8589-cf41e46470c3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f313ba2f-1437-408e-9973-eeb11d6c56a0" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_2ea5a6e9-25c3-4069-8589-cf41e46470c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_32ef77eb-3e82-4546-9cc8-e0c122da6947" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f313ba2f-1437-408e-9973-eeb11d6c56a0" xlink:to="loc_us-gaap_EmployeeStockOptionMember_32ef77eb-3e82-4546-9cc8-e0c122da6947" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_e949cb6e-22b5-4627-9b7a-ee547dbb712c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f313ba2f-1437-408e-9973-eeb11d6c56a0" xlink:to="loc_us-gaap_EmployeeStockMember_e949cb6e-22b5-4627-9b7a-ee547dbb712c" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/SupplementalFinancialInformationNoncashActivitiesDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#SupplementalFinancialInformationNoncashActivitiesDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/SupplementalFinancialInformationNoncashActivitiesDetails" xlink:type="extended" id="i35977d4e1b4e499abb2d6d701b316498_SupplementalFinancialInformationNoncashActivitiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_SupplementalFinancialInformationLineItems_39ccece1-2e68-4303-81ce-f58bf319efa9" xlink:href="cmcsa-20210930.xsd#cmcsa_SupplementalFinancialInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_c3dab9e2-b750-432d-a51e-39c8ea1f2e0d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_SupplementalFinancialInformationLineItems_39ccece1-2e68-4303-81ce-f58bf319efa9" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_c3dab9e2-b750-432d-a51e-39c8ea1f2e0d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_1b64c237-a3b2-448b-96cb-853c1ff92be9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_SupplementalFinancialInformationLineItems_39ccece1-2e68-4303-81ce-f58bf319efa9" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_1b64c237-a3b2-448b-96cb-853c1ff92be9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_0482609b-daa3-4f25-b574-8ada74485d9d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_SupplementalFinancialInformationLineItems_39ccece1-2e68-4303-81ce-f58bf319efa9" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_0482609b-daa3-4f25-b574-8ada74485d9d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CapitalCostsIncurredButNotYetPaid_e96a8922-4265-4c91-ab8a-658e14a18c6d" xlink:href="cmcsa-20210930.xsd#cmcsa_CapitalCostsIncurredButNotYetPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_SupplementalFinancialInformationLineItems_39ccece1-2e68-4303-81ce-f58bf319efa9" xlink:to="loc_cmcsa_CapitalCostsIncurredButNotYetPaid_e96a8922-4265-4c91-ab8a-658e14a18c6d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsPayableCurrentAndNoncurrent_3b273d08-d46c-4204-bc1b-7ceed9e31f6f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DividendsPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_SupplementalFinancialInformationLineItems_39ccece1-2e68-4303-81ce-f58bf319efa9" xlink:to="loc_us-gaap_DividendsPayableCurrentAndNoncurrent_3b273d08-d46c-4204-bc1b-7ceed9e31f6f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsPayableAmountPerShare_39fdb2c2-87d0-41ec-9a34-8542a57d9d43" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DividendsPayableAmountPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_SupplementalFinancialInformationLineItems_39ccece1-2e68-4303-81ce-f58bf319efa9" xlink:to="loc_us-gaap_DividendsPayableAmountPerShare_39fdb2c2-87d0-41ec-9a34-8542a57d9d43" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_SupplementalFinancialInformationTable_8f76d523-e99a-4ca0-91a7-6a37fd866a79" xlink:href="cmcsa-20210930.xsd#cmcsa_SupplementalFinancialInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_cmcsa_SupplementalFinancialInformationLineItems_39ccece1-2e68-4303-81ce-f58bf319efa9" xlink:to="loc_cmcsa_SupplementalFinancialInformationTable_8f76d523-e99a-4ca0-91a7-6a37fd866a79" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_LeaseArrangementsAxis_3e94e1a2-c8d0-45da-85bb-2f160718caff" xlink:href="cmcsa-20210930.xsd#cmcsa_LeaseArrangementsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_cmcsa_SupplementalFinancialInformationTable_8f76d523-e99a-4ca0-91a7-6a37fd866a79" xlink:to="loc_cmcsa_LeaseArrangementsAxis_3e94e1a2-c8d0-45da-85bb-2f160718caff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_LeaseArrangementsDomain_3e94e1a2-c8d0-45da-85bb-2f160718caff_default" xlink:href="cmcsa-20210930.xsd#cmcsa_LeaseArrangementsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_cmcsa_LeaseArrangementsAxis_3e94e1a2-c8d0-45da-85bb-2f160718caff" xlink:to="loc_cmcsa_LeaseArrangementsDomain_3e94e1a2-c8d0-45da-85bb-2f160718caff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_LeaseArrangementsDomain_d68fdf52-4860-43fb-b6d4-288201697bc2" xlink:href="cmcsa-20210930.xsd#cmcsa_LeaseArrangementsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_cmcsa_LeaseArrangementsAxis_3e94e1a2-c8d0-45da-85bb-2f160718caff" xlink:to="loc_cmcsa_LeaseArrangementsDomain_d68fdf52-4860-43fb-b6d4-288201697bc2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_UniversalBeijingResortMember_36711c8a-26d7-4740-86d1-f074ae9857ed" xlink:href="cmcsa-20210930.xsd#cmcsa_UniversalBeijingResortMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_LeaseArrangementsDomain_d68fdf52-4860-43fb-b6d4-288201697bc2" xlink:to="loc_cmcsa_UniversalBeijingResortMember_36711c8a-26d7-4740-86d1-f074ae9857ed" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.comcast.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="cmcsa-20210930.xsd#CommitmentsandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="extended" id="i232ab5388ced40d2a78ae10f164922b3_CommitmentsandContingenciesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CommitmentAndContingenciesLineItems_d4cab61a-eca0-4b0d-881d-3f1de446413d" xlink:href="cmcsa-20210930.xsd#cmcsa_CommitmentAndContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_PaymentsForRepurchaseOfRedeemableSubsidiaryPreferredStock_0a60475e-f1ef-404c-8aac-f5b40a32cea0" xlink:href="cmcsa-20210930.xsd#cmcsa_PaymentsForRepurchaseOfRedeemableSubsidiaryPreferredStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cmcsa_CommitmentAndContingenciesLineItems_d4cab61a-eca0-4b0d-881d-3f1de446413d" xlink:to="loc_cmcsa_PaymentsForRepurchaseOfRedeemableSubsidiaryPreferredStock_0a60475e-f1ef-404c-8aac-f5b40a32cea0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_CommitmentAndContingenciesTable_5106bea0-ed54-42bb-919a-77e715671dd6" xlink:href="cmcsa-20210930.xsd#cmcsa_CommitmentAndContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_cmcsa_CommitmentAndContingenciesLineItems_d4cab61a-eca0-4b0d-881d-3f1de446413d" xlink:to="loc_cmcsa_CommitmentAndContingenciesTable_5106bea0-ed54-42bb-919a-77e715671dd6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_b911f6eb-18bb-4a34-89e2-588fac71ef7d" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_cmcsa_CommitmentAndContingenciesTable_5106bea0-ed54-42bb-919a-77e715671dd6" xlink:to="loc_dei_LegalEntityAxis_b911f6eb-18bb-4a34-89e2-588fac71ef7d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_b911f6eb-18bb-4a34-89e2-588fac71ef7d_default" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_b911f6eb-18bb-4a34-89e2-588fac71ef7d" xlink:to="loc_dei_EntityDomain_b911f6eb-18bb-4a34-89e2-588fac71ef7d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_39f602ea-af81-44fa-8a34-7bfd5e938a72" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_b911f6eb-18bb-4a34-89e2-588fac71ef7d" xlink:to="loc_dei_EntityDomain_39f602ea-af81-44fa-8a34-7bfd5e938a72" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_NbcuniversalEnterpriseMember_9b8b5739-d17f-472c-af06-5526e6cff2c0" xlink:href="cmcsa-20210930.xsd#cmcsa_NbcuniversalEnterpriseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_39f602ea-af81-44fa-8a34-7bfd5e938a72" xlink:to="loc_cmcsa_NbcuniversalEnterpriseMember_9b8b5739-d17f-472c-af06-5526e6cff2c0" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
