Stands for "Application Program Interface," though it is sometimes referred to as an "Application Programming Interface." An API is a set of commands, functions, and protocols which programmers can use when building software for a specific operating system. The API allows programmers to use predefined functions to interact with the operating system, instead of writing them from scratch.
All computer operating systems, such as Windows, Unix, and the Mac OS, provide an application program interface for programmers. APIs are also used by video game consoles and other hardware devices that can run software programs. While the API makes the programmer's job easier, it also benefits the end user, since it ensures all programs using the same API will have a similar user interface.