A
Application Programming Interface (API)
Definition
An interface that a software program implements in order to allow other software to interact with it, much in the same way that software might implement a user interface in order to allow humans to use it. APIs are implemented by applications, libraries and operating systems to define how other software can make calls to or request services from them.[1][2][3] An API determines the vocabulary and calling conventions the programmer should employ to use the services. It may include specifications for routines, data structures, object classes, and protocols used to communicate between the consumer and implementer of the API.
Reference:
Wikipedia