API

An application programming interface (API) is a source code interface that a computer system or program library provides in order to support requests for services to be made of it by a computer program. An API is often a part of a software development kit (SDK).

The term API is used in two related senses: A coherent interface consisting of several classes or several sets of related functions or procedures, or a single entry point such as a method, function or procedure. One popular API is the Single UNIX Specification.

More Information
An API differs from an application binary interface in that it is specified in terms of a programming language that can be compiled when an application is built, rather than an explicit low level description of how data is laid out in memory.

The software that provides the functionality described by an API is said to be an implementation of the API. The API itself is abstract, in that it specifies an interface and does not get involved with implementation details.