In computer programming, an application programming interface (API) is a set of subroutine definitions, protocols, and tools for building application software. In general terms,** it's a set of clearly defined methods** of communication between various software components. A good API makes it easier to develop a computer program by providing all the building blocks, which are then put together by the programmer. An API may be for a web-based system, operating system, database system, computer hardware, or software library. An API specification can take many forms, but often includes specifications for routines, data structures, object classes, variables, or remote calls. POSIX, Microsoft Windows API, the C++ Standard Template Library, and Java APIs are examples of different forms of APIs. Documentation for the API is usually provided to facilitate usage. The status of APIs in intellectual property law is controversial.
In general, abstraction is implemented by what is generically termed an Application Programming Interface (API). API is a somewhat nebulous term that means different things in the context of various programming endavours. Fundamentally, a programmer designs a set of functions and documents their interface and functionality with the principle that the actual implementation providing the API is opaque.