The best replacement for api is sdk.

The best alternatives to api are rated as following:
★★★★★  sdk
★★★★  neve
★★★  web service
★★  library
★★  ayp
★★  ssl
★★  framework
★★  spi
★★  soa
★★  specific gravity
★  oop
★  apis
★  ansi
★  web services
★  gui
★  summit
★  service
★  dll
★  rss
★  protocol
★  great river
★  ftp
  seachem
  asme
  mfc
  system call
  nutrafin
  apk
  daking
  wcf
  plugin
  psi
  acea
  astm
  soap
  rest
  naga air
  data feed
  aqi
  cea
  object oriented programming
  wsdl
  directshow

Your alternative for api is not listed? Suggest!
Speaking about api, one probably shouldn't forget about mvc.

Other search terms related to api and their ratings are:
★★★★★ mvc
★★ authentication
★★ entity framework
★★ oauth
★ login
★ access token
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Google Trends

Scale is based on the average worldwide traffic of api in all years. Click on graph for more.
api
sdk
neve