(Application Programming Interface) A language and message format used by an application program to communicate with the operating system or some other control program such as a database management system (DBMS) or communications protocol. APIs are implemented by writing function calls in the program, which provide the linkage to the required subroutine for execution. Thus, an API implies that some program module is available in the computer to perform the operation or that it must be linked into the existing program to perform the tasks.

Plenty of API Programming Is Done
Understanding an API is a major part of what a programmer does. Except for writing the business logic that performs the actual data processing, all the rest of the programming is writing the code to communicate with the functions in the operating system and other system software such as a DBMS.

The APIs for operating systems can be daunting, especially the calls to the user interface routines to display contents on the screen. There are more than a thousand API calls in a full-blown operating system such as Windows, Mac or Unix. See function and interface.