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

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2021 Workiva-->
<!--r:17bfb22c-a843-4a2d-94c4-6d68fe97503c,g:b871df9b-e153-4e81-afc2-faaeceef4ff1-->
<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/DocumentandEntityInformation" xlink:type="simple" xlink:href="cmcsa-20210128.xsd#DocumentandEntityInformation"/>
  <link:definitionLink xlink:role="http://www.comcast.com/role/DocumentandEntityInformation" xlink:type="extended" id="idf646c12cad34a0b946b70827db8f2fb_DocumentandEntityInformation">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_34eba3e7-acf1-4832-91cc-e29ff1e21274" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_DocumentType_34eba3e7-acf1-4832-91cc-e29ff1e21274" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_b387124f-7e37-44f5-a2d9-d77521d57270" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_AmendmentFlag_b387124f-7e37-44f5-a2d9-d77521d57270" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_2891735d-346e-4f88-b4bb-f6d4b9c00e30" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_DocumentPeriodEndDate_2891735d-346e-4f88-b4bb-f6d4b9c00e30" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_d2ba133c-0329-40f1-8003-36b451d2cc07" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityRegistrantName_d2ba133c-0329-40f1-8003-36b451d2cc07" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_52be79d6-af4d-48a4-b79d-661ab1f104c4" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityCentralIndexKey_52be79d6-af4d-48a4-b79d-661ab1f104c4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_1d7db95c-ba22-4c9c-ac9d-98a3def83117" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityIncorporationStateCountryCode_1d7db95c-ba22-4c9c-ac9d-98a3def83117" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_ba28e6f9-68a8-48bd-a996-fac650675767" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityFileNumber_ba28e6f9-68a8-48bd-a996-fac650675767" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_455c50ae-f151-4f98-bd5d-72898cf9bd93" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityTaxIdentificationNumber_455c50ae-f151-4f98-bd5d-72898cf9bd93" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_26d82737-2e13-4225-9a76-9ce224c2476b" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityAddressAddressLine1_26d82737-2e13-4225-9a76-9ce224c2476b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_63c66d3a-237f-4740-8677-d0a15284d6d9" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityAddressCityOrTown_63c66d3a-237f-4740-8677-d0a15284d6d9" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_6d7545b6-14a7-4abc-9378-56bc21da18c2" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityAddressStateOrProvince_6d7545b6-14a7-4abc-9378-56bc21da18c2" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_35f6c9e0-b3db-4978-a369-4902c9503e43" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityAddressPostalZipCode_35f6c9e0-b3db-4978-a369-4902c9503e43" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_c34bd84c-cade-458d-8aac-3495f4219b86" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_CityAreaCode_c34bd84c-cade-458d-8aac-3495f4219b86" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_c2cbb43c-75ca-4c37-a4b0-74b444636cad" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_LocalPhoneNumber_c2cbb43c-75ca-4c37-a4b0-74b444636cad" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_WrittenCommunications_fdfd3ee2-6392-43ac-aaae-7ced3708ea5e" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_WrittenCommunications"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_WrittenCommunications_fdfd3ee2-6392-43ac-aaae-7ced3708ea5e" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SolicitingMaterial_c8a74add-b893-40f3-902d-cbe60d44921d" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_SolicitingMaterial"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_SolicitingMaterial_c8a74add-b893-40f3-902d-cbe60d44921d" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementTenderOffer_b52c5a82-eb8a-4a12-a3cd-b583aba20172" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_PreCommencementTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_PreCommencementTenderOffer_b52c5a82-eb8a-4a12-a3cd-b583aba20172" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementIssuerTenderOffer_d192e678-a405-4161-8438-aa2ed0cdad3d" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_PreCommencementIssuerTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_PreCommencementIssuerTenderOffer_d192e678-a405-4161-8438-aa2ed0cdad3d" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_977b0a5d-f2b2-44c9-8a0b-fda754e2dee5" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_Security12bTitle_977b0a5d-f2b2-44c9-8a0b-fda754e2dee5" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_fd71caf5-d61a-4f5f-b0a2-4d785f2e9c54" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_TradingSymbol_fd71caf5-d61a-4f5f-b0a2-4d785f2e9c54" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_e8bd1d0d-8ccc-48d4-b0d7-a814be31eac7" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_SecurityExchangeName_e8bd1d0d-8ccc-48d4-b0d7-a814be31eac7" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_a330d7f9-f964-4073-bdc8-e2e6e6806c63" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_EntityEmergingGrowthCompany_a330d7f9-f964-4073-bdc8-e2e6e6806c63" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_f3af8f07-00d6-448a-94c8-5a1575e0833f" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_b5a3a95a-8bd9-439c-9791-fb8b424409d3" xlink:to="loc_dei_DocumentInformationTable_f3af8f07-00d6-448a-94c8-5a1575e0833f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_37a7c53b-3c39-4634-b167-c52e89d42ae6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_f3af8f07-00d6-448a-94c8-5a1575e0833f" xlink:to="loc_us-gaap_StatementClassOfStockAxis_37a7c53b-3c39-4634-b167-c52e89d42ae6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_37a7c53b-3c39-4634-b167-c52e89d42ae6_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_37a7c53b-3c39-4634-b167-c52e89d42ae6" xlink:to="loc_us-gaap_ClassOfStockDomain_37a7c53b-3c39-4634-b167-c52e89d42ae6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_37a7c53b-3c39-4634-b167-c52e89d42ae6" xlink:to="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_ee3c182a-0d2c-4f4f-9a81-676037e3beb5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_us-gaap_CommonClassAMember_ee3c182a-0d2c-4f4f-9a81-676037e3beb5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes0.250percentDue2027Member_b596925f-02cc-4124-b228-5d73905c415a" xlink:href="cmcsa-20210128.xsd#cmcsa_Notes0.250percentDue2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_cmcsa_Notes0.250percentDue2027Member_b596925f-02cc-4124-b228-5d73905c415a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes1.500percentDue2029Member_73955f53-ea25-45ec-8b17-de797ccbadea" xlink:href="cmcsa-20210128.xsd#cmcsa_Notes1.500percentDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_cmcsa_Notes1.500percentDue2029Member_73955f53-ea25-45ec-8b17-de797ccbadea" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes0.750percentdue2032Member_38cc45c7-e68a-4064-89de-13597cd716e7" xlink:href="cmcsa-20210128.xsd#cmcsa_Notes0.750percentdue2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_cmcsa_Notes0.750percentdue2032Member_38cc45c7-e68a-4064-89de-13597cd716e7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes1.875percentdue2036Member_6ec171ca-5fbb-4ede-b3ee-fb2a36ab6d80" xlink:href="cmcsa-20210128.xsd#cmcsa_Notes1.875percentdue2036Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_cmcsa_Notes1.875percentdue2036Member_6ec171ca-5fbb-4ede-b3ee-fb2a36ab6d80" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes1.250percentdue2040Member_3177a17d-7f68-404f-b5b5-581d0ccf51ae" xlink:href="cmcsa-20210128.xsd#cmcsa_Notes1.250percentdue2040Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_cmcsa_Notes1.250percentdue2040Member_3177a17d-7f68-404f-b5b5-581d0ccf51ae" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_GuaranteedNotes9.455PercentDue2022Member_474d0eca-06e3-436c-98db-7380c8165664" xlink:href="cmcsa-20210128.xsd#cmcsa_GuaranteedNotes9.455PercentDue2022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_cmcsa_GuaranteedNotes9.455PercentDue2022Member_474d0eca-06e3-436c-98db-7380c8165664" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_Notes5.50PercentDue2029Member_23e86354-3e65-4fb9-a49e-31ece52c37b3" xlink:href="cmcsa-20210128.xsd#cmcsa_Notes5.50PercentDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_cmcsa_Notes5.50PercentDue2029Member_23e86354-3e65-4fb9-a49e-31ece52c37b3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cmcsa_ExchangeableSubordinatedDebentures2.0PercentDue2029Member_9fda1666-35c1-48ac-9d36-d6745139d1de" xlink:href="cmcsa-20210128.xsd#cmcsa_ExchangeableSubordinatedDebentures2.0PercentDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9d2d894f-6440-40f9-9533-9f445491e827" xlink:to="loc_cmcsa_ExchangeableSubordinatedDebentures2.0PercentDue2029Member_9fda1666-35c1-48ac-9d36-d6745139d1de" xlink:type="arc" order="8"/>
  </link:definitionLink>
</link:linkbase>
