An application programming interface (API) is a protocol intended to be used as an interface by software components to communicate with each other. An API may include specifications for routines, data structures, object classes, and variables. An API specification can take many forms, including an International Standard such as POSIX, vendor documentation such as the Microsoft Windows API, the libraries of a programming language, e.g. Standard Template Library in C++ or Java API. Garter predicts that by 2014 75% of Fortune 500 enterprises will open an API.
An API differs from an application binary interface (ABI) in that an API is source code based while an ABI is a binary interface. For instance POSIX is an API, while the Linux Standard Base is an ABI.
Read more about API: Detailed Explanation, Web APIs, Implementations, Release Policies, APIs and Copyrights, API Examples, Language Bindings and Interface Generators