what is an api?

API is application program interface.

In easier terms, it is a function,method or procedure(whatever term you like in programming)which can have direct access to routines carried out by operating system (or sometimes in runtime environment like JVM or DotNet CLR)

As a baseball analogy, an API is the rule book. If you did not know anything about the inner workings of baseball, you would get a copy of the rule book. This book would show you what each player's (the functions or classes) responsibilites are, how the players are supposed to interact with each other (the signals or methods), and what the players need to play the game such as baseballs, gloves, and bats (the variables and inputs). Once you know the rules (the API), you should be able to go out and build your own baseball team (program) or at least be able to play on another baseball team (a library). Or you could go read the wikipedia article on API.

Application Program Interface

An application programming interface (API) is a set of definitions of the ways one piece of computer software communicates with another. It is a method of achieving abstraction, usually (but not necessarily) between lower-level and higher-level software.

Application Program Interface.

It's a structured way in which you can interact with the internals of a program without specific knowledge of the way in which those internals work.

Its Application Program Interface. this is the concept of Microsoft.