Application Program Interface, which is a set of calling conventions by which an application accesses an operating system and other services. An API is defined in the source code and provides a level of abstraction between an application and the kernel (or other privileged utilities) to ensure the portability of the code.
An API can also provide an interface between a high-level language and lower-level utilities and services written without consideration for the calling conventions supported by compiled languages. In this case, the API can translate the parameter lists from one format to another and the interpret call-by-value and call-by-reference arguments in one or both directions.