An acronym for Application Programming Interface. An API is an interface that a software application implements to enable other software applications to interact with it. APIs are implemented by applications, libraries and operating systems to define how other software can make calls to or request services from them. An API determines the method 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 user and implementer of the API.