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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e8514-58d5-71e1-a709-47ddfc86233b,g:efc25606-a1aa-4e78-b17b-c3619e8390a7-->
<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://xbrl.sec.gov/oef/role/SeriesOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2026/oef-2026.xsd#SeriesOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/oef/role/SeriesOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_S000005965Member_019e8514-58d5-75e2-bdfb-6b67613cb0b8" xlink:href="ck0000773674-20260729.xsd#ck0000773674_S000005965Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain" xlink:to="loc_ck0000773674_S000005965Member_019e8514-58d5-75e2-bdfb-6b67613cb0b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_S000005966Member_019e8514-58d5-79ff-88fb-fb049d241c7a" xlink:href="ck0000773674-20260729.xsd#ck0000773674_S000005966Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain" xlink:to="loc_ck0000773674_S000005966Member_019e8514-58d5-79ff-88fb-fb049d241c7a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_S000005968Member_019e8514-58d5-737d-a20d-6bf78fe175b0" xlink:href="ck0000773674-20260729.xsd#ck0000773674_S000005968Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain" xlink:to="loc_ck0000773674_S000005968Member_019e8514-58d5-737d-a20d-6bf78fe175b0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_S000005970Member_019e8514-58d5-7907-b5e7-7ff86d62861d" xlink:href="ck0000773674-20260729.xsd#ck0000773674_S000005970Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain" xlink:to="loc_ck0000773674_S000005970Member_019e8514-58d5-7907-b5e7-7ff86d62861d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_S000005969Member_019e8514-58d5-7b6a-bca7-7128e2caf79d" xlink:href="ck0000773674-20260729.xsd#ck0000773674_S000005969Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain" xlink:to="loc_ck0000773674_S000005969Member_019e8514-58d5-7b6a-bca7-7128e2caf79d" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/rr/role/ProspectusOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2026/oef-2026.xsd#ProspectusOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/rr/role/ProspectusOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentDomain" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_AmericanCenturyGovernmentIncomeTrustMember_019e8514-58d5-76c1-bcdd-492b4d57f35c" xlink:href="ck0000773674-20260729.xsd#ck0000773674_AmericanCenturyGovernmentIncomeTrustMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentDomain" xlink:to="loc_ck0000773674_AmericanCenturyGovernmentIncomeTrustMember_019e8514-58d5-76c1-bcdd-492b4d57f35c" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/oef/role/ClassOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2026/oef-2026.xsd#ClassOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/oef/role/ClassOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_oef_ShareClassDomain" xlink:href="https://xbrl.sec.gov/oef/2026/oef-2026.xsd#oef_ShareClassDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016429Member_019e8514-58d5-7ae1-a497-759bc42c8887" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016429Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016429Member_019e8514-58d5-7ae1-a497-759bc42c8887" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016430Member_019e8514-58d5-7eba-b849-bf8df3da386a" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016430Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016430Member_019e8514-58d5-7eba-b849-bf8df3da386a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000189731Member_019e8514-58d5-76be-a6df-9e56bad065de" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000189731Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000189731Member_019e8514-58d5-76be-a6df-9e56bad065de" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016431Member_019e8514-58d5-75be-a034-5940dbe27e91" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016431Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016431Member_019e8514-58d5-75be-a034-5940dbe27e91" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088034Member_019e8514-58d5-7853-8d07-cea30a8acb9f" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088034Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088034Member_019e8514-58d5-7853-8d07-cea30a8acb9f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000051003Member_019e8514-58d5-70e3-bca5-0bab784f97a2" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000051003Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000051003Member_019e8514-58d5-70e3-bca5-0bab784f97a2" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000051002Member_019e8514-58d5-7168-9a87-c61fd1cbe71e" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000051002Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000051002Member_019e8514-58d5-7168-9a87-c61fd1cbe71e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016435Member_019e8514-58d5-7360-9c9e-6a9c495e3727" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016435Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016435Member_019e8514-58d5-7360-9c9e-6a9c495e3727" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000189732Member_019e8514-58d5-7f1c-8c06-99fd2ac5ce68" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000189732Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000189732Member_019e8514-58d5-7f1c-8c06-99fd2ac5ce68" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016436Member_019e8514-58d5-7136-afdc-e8737dc04335" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016436Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016436Member_019e8514-58d5-7136-afdc-e8737dc04335" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088036Member_019e8514-58d5-7541-ae86-6cc0f4fa0ac5" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088036Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088036Member_019e8514-58d5-7541-ae86-6cc0f4fa0ac5" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088037Member_019e8514-58d5-7d0c-8626-ce5a7531937b" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088037Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088037Member_019e8514-58d5-7d0c-8626-ce5a7531937b" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088035Member_019e8514-58d5-7d51-925f-052257d04396" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088035Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088035Member_019e8514-58d5-7d51-925f-052257d04396" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016440Member_019e8514-58d5-79ff-b77e-0caa08fe9609" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016440Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016440Member_019e8514-58d5-79ff-b77e-0caa08fe9609" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000189733Member_019e8514-58d5-7f35-aaf7-200ae360ce12" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000189733Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000189733Member_019e8514-58d5-7f35-aaf7-200ae360ce12" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016441Member_019e8514-58d5-74ef-b76f-e301e47cd317" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016441Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016441Member_019e8514-58d5-74ef-b76f-e301e47cd317" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088041Member_019e8514-58d5-7d6f-b15a-3932c7d01c4e" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088041Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088041Member_019e8514-58d5-7d6f-b15a-3932c7d01c4e" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088042Member_019e8514-58d5-71ae-a819-1053e1f1b271" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088042Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088042Member_019e8514-58d5-71ae-a819-1053e1f1b271" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088040Member_019e8514-58d5-7b34-ad20-eaea41d77c9c" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088040Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088040Member_019e8514-58d5-7b34-ad20-eaea41d77c9c" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016437Member_019e8514-58d5-7591-8eed-22856696e14e" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016437Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016437Member_019e8514-58d5-7591-8eed-22856696e14e" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000181796Member_019e8514-58d5-7d1a-a0b9-f26466e57a4a" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000181796Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000181796Member_019e8514-58d5-7d1a-a0b9-f26466e57a4a" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000181797Member_019e8514-58d5-7c24-8e0d-2ffd7c751bc2" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000181797Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000181797Member_019e8514-58d5-7c24-8e0d-2ffd7c751bc2" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016438Member_019e8514-58d5-78e3-91e1-e4576a7756e4" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016438Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016438Member_019e8514-58d5-78e3-91e1-e4576a7756e4" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088038Member_019e8514-58d5-7da4-aa76-f9b8109f70c9" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088038Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088038Member_019e8514-58d5-7da4-aa76-f9b8109f70c9" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000088039Member_019e8514-58d5-7fe8-810b-f09a874bb596" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000088039Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000088039Member_019e8514-58d5-7fe8-810b-f09a874bb596" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000016439Member_019e8514-58d5-713b-8c97-88c54bf763fd" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000016439Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000016439Member_019e8514-58d5-713b-8c97-88c54bf763fd" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000191079Member_019e8514-58d5-78d4-8a76-b12cdfd0cbd3" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000191079Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000191079Member_019e8514-58d5-78d4-8a76-b12cdfd0cbd3" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_C000191080Member_019e8514-58d5-75a5-bcd3-40c96afbebff" xlink:href="ck0000773674-20260729.xsd#ck0000773674_C000191080Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_ShareClassDomain" xlink:to="loc_ck0000773674_C000191080Member_019e8514-58d5-75a5-bcd3-40c96afbebff" xlink:type="arc" order="28"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/rr/role/RiskOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2026/oef-2026.xsd#RiskOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/rr/role/RiskOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_oef_AllRisksMember" xlink:href="https://xbrl.sec.gov/oef/2026/oef-2026.xsd#oef_AllRisksMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_InterestRateRisksMember_019e8514-58d5-75f1-88cc-cc8e92777b17" xlink:href="ck0000773674-20260729.xsd#ck0000773674_InterestRateRisksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_InterestRateRisksMember_019e8514-58d5-75f1-88cc-cc8e92777b17" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_PrepaymentAndExtensionRiskMember_019e8514-58d5-7de7-9b85-503880308af4" xlink:href="ck0000773674-20260729.xsd#ck0000773674_PrepaymentAndExtensionRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_PrepaymentAndExtensionRiskMember_019e8514-58d5-7de7-9b85-503880308af4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_DerivativesRiskMember_019e8514-58d5-764f-9fbb-770f8f7fbf76" xlink:href="ck0000773674-20260729.xsd#ck0000773674_DerivativesRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_DerivativesRiskMember_019e8514-58d5-764f-9fbb-770f8f7fbf76" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_MarketRiskMember_019e8514-58d5-7cbd-91d6-b3659b40696e" xlink:href="ck0000773674-20260729.xsd#ck0000773674_MarketRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_MarketRiskMember_019e8514-58d5-7cbd-91d6-b3659b40696e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_RedemptionRiskMember_019e8514-58d5-7119-9310-eff7e4b560a2" xlink:href="ck0000773674-20260729.xsd#ck0000773674_RedemptionRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_RedemptionRiskMember_019e8514-58d5-7119-9310-eff7e4b560a2" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_InflationIndexedBondsRiskMember_019e8514-58d5-7285-8c1a-a83a22aff8d8" xlink:href="ck0000773674-20260729.xsd#ck0000773674_InflationIndexedBondsRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_InflationIndexedBondsRiskMember_019e8514-58d5-7285-8c1a-a83a22aff8d8" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_CreditRisksMember_019e8514-58d5-7edb-b75c-4d23cb9e34e7" xlink:href="ck0000773674-20260729.xsd#ck0000773674_CreditRisksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_CreditRisksMember_019e8514-58d5-7edb-b75c-4d23cb9e34e7" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_HighYieldRiskMember_019e8514-58d5-7f5b-8873-88ba4415bd3f" xlink:href="ck0000773674-20260729.xsd#ck0000773674_HighYieldRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_HighYieldRiskMember_019e8514-58d5-7f5b-8873-88ba4415bd3f" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_LiquidityRiskMember_019e8514-58d5-754d-a791-1339b3f91f42" xlink:href="ck0000773674-20260729.xsd#ck0000773674_LiquidityRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_LiquidityRiskMember_019e8514-58d5-754d-a791-1339b3f91f42" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_ForeignSecuritiesRiskMember_019e8514-58d5-7653-9dcc-04fe15fab1c2" xlink:href="ck0000773674-20260729.xsd#ck0000773674_ForeignSecuritiesRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_ForeignSecuritiesRiskMember_019e8514-58d5-7653-9dcc-04fe15fab1c2" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_BankLoanRiskMember_019e8514-58d5-7ab3-adb8-989d65895925" xlink:href="ck0000773674-20260729.xsd#ck0000773674_BankLoanRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_BankLoanRiskMember_019e8514-58d5-7ab3-adb8-989d65895925" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_CollateralizedObligationsRiskMember_019e8514-58d5-76e5-a92a-2e891e5b08de" xlink:href="ck0000773674-20260729.xsd#ck0000773674_CollateralizedObligationsRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_CollateralizedObligationsRiskMember_019e8514-58d5-76e5-a92a-2e891e5b08de" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_WhenIssuedAndForwardCommitmentRiskMember_019e8514-58d5-75f8-a49f-22dc89eda0ce" xlink:href="ck0000773674-20260729.xsd#ck0000773674_WhenIssuedAndForwardCommitmentRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_WhenIssuedAndForwardCommitmentRiskMember_019e8514-58d5-75f8-a49f-22dc89eda0ce" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_MortgageAndAssetBackedSecuritiesRiskMember_019e8514-58d5-727e-877d-f2a57b1d7648" xlink:href="ck0000773674-20260729.xsd#ck0000773674_MortgageAndAssetBackedSecuritiesRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_MortgageAndAssetBackedSecuritiesRiskMember_019e8514-58d5-727e-877d-f2a57b1d7648" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_MortgageDollarRollsRiskMember_019e8514-58d5-707b-97b5-40b48486c364" xlink:href="ck0000773674-20260729.xsd#ck0000773674_MortgageDollarRollsRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_AllRisksMember" xlink:to="loc_ck0000773674_MortgageDollarRollsRiskMember_019e8514-58d5-707b-97b5-40b48486c364" xlink:type="arc" order="23"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/rr/role/PerformanceMeasureOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2026/oef-2026.xsd#PerformanceMeasureOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/rr/role/PerformanceMeasureOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_oef_PerformanceMeasureDomain" xlink:href="https://xbrl.sec.gov/oef/2026/oef-2026.xsd#oef_PerformanceMeasureDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_BloombergU.S.AggregateBondIndexMember_019e8514-58d5-7116-aff6-3ecc0e2ff2ee" xlink:href="ck0000773674-20260729.xsd#ck0000773674_BloombergU.S.AggregateBondIndexMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_PerformanceMeasureDomain" xlink:to="loc_ck0000773674_BloombergU.S.AggregateBondIndexMember_019e8514-58d5-7116-aff6-3ecc0e2ff2ee" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_BloombergUSGNMAIndexMember_019e8514-58d5-79e6-9152-dea8473d072e" xlink:href="ck0000773674-20260729.xsd#ck0000773674_BloombergUSGNMAIndexMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_PerformanceMeasureDomain" xlink:to="loc_ck0000773674_BloombergUSGNMAIndexMember_019e8514-58d5-79e6-9152-dea8473d072e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_BloombergUSGovernmentMBSIndexMember_019e8514-58d5-724b-b5c0-e0179bb8684c" xlink:href="ck0000773674-20260729.xsd#ck0000773674_BloombergUSGovernmentMBSIndexMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_PerformanceMeasureDomain" xlink:to="loc_ck0000773674_BloombergUSGovernmentMBSIndexMember_019e8514-58d5-724b-b5c0-e0179bb8684c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_BloombergUS13YearGovernmentBondIndexMember_019e8514-58d5-72ae-a5f9-da5b071016a7" xlink:href="ck0000773674-20260729.xsd#ck0000773674_BloombergUS13YearGovernmentBondIndexMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_PerformanceMeasureDomain" xlink:to="loc_ck0000773674_BloombergUS13YearGovernmentBondIndexMember_019e8514-58d5-72ae-a5f9-da5b071016a7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ck0000773674_BloombergUSTreasuryInflationProtectedSecuritiesTIPSIndexMember_019e8514-58d5-7d58-9798-4e5a0da68704" xlink:href="ck0000773674-20260729.xsd#ck0000773674_BloombergUSTreasuryInflationProtectedSecuritiesTIPSIndexMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_oef_PerformanceMeasureDomain" xlink:to="loc_ck0000773674_BloombergUSTreasuryInflationProtectedSecuritiesTIPSIndexMember_019e8514-58d5-7d58-9798-4e5a0da68704" xlink:type="arc" order="7"/>
  </link:definitionLink>
</link:linkbase>
