API

An Application Programmer Interface (also called an API) is a way for a computer program to communicate what services it is willing to provide to other programs.

A program that calls an API is one which uses the services provided. A program that defines an API is one which provides the services.

An API is usually contrasted to a User Interface (UI) which is a service provided to a person who is using the computer to accomplish tasks. Typically UIs are either text based (CHUI) or graphics based (GUI).

Generally, an API will list what services are provided, what inputs are needed, and what output is produced.