Source : Free On-Line Dictionary of Computing

Application Program Interface
     
         (API, or "application programming interface")
        The interface (calling conventions) by which an {application
        program} accesses {operating system} and other services.  An
        API is defined at {source code} level and provides a level of
        {abstraction} between the 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 which were
        written without consideration for the {calling conventions}
        supported by compiled languages.  In this case, the API's main
        task may be the translation of parameter lists from one format
        to another and the interpretation of {call-by-value} and
        {call-by-reference} arguments in one or both directions.
     
        (1995-02-15)