Definition: API

(Application Programming Interface) A message format that enables software to communicate with other software. Applications connect to the operating system, database management system, social media and countless other platforms via an API.

There are countless APIs. A financial app uses an API to access stock market data on a server. A weather app uses an API to connect to a weather source. Fitness apps use an API to interact with the pulse monitor in a phone or watch.

OS Function Calls
Applications communicate with the OS with function calls (lines of code) to display windows, menus, buttons and data. Other than the business logic, programmers do a lot of API coding, with each environment (Windows, Mac, iOS, Android and Linux) dramatically different from the other. An iPhone programmer has to learn an entirely new set of codes and commands to be an Android programmer. See integrated development environment, function and interface.

APIs Become Obsolete
Operating systems are upgraded all the time, and vendors discontinue support for old routines after a while. When old APIs no longer work with the new OS, the application has to be revised. Business organizations can put off the latest OS for a while but developers must be compatible with new OS versions because they serve everyone.