API / ABI / POSIX

API => Application Programming Interface

An application programming interface (API) is a set of routines, data structures, object classes and/or protocols provided by libraries and/or operating system services in order to support the building of applications.

The API itself is largely abstract in that it specifies an interface and controls the behavior of the objects specified in that interface. The software that provides the functionality described by an API is said to be an implementation of the API. An API is typically defined in terms of the programming language used to build an application. The related term, ABI (Application Binary Interface), is a lower level definition concerning details at the Assembly language level. For example, the Linux Standard Base is an ABI, while POSIX is an API.

POSIX => Portable Operating System Interface

The POSIX standard defines an API that allows a wide range of common computing functions to be written such that they may operate on many different systems (Mac OS X and various BSDs implement this interface); however, making use of this requires re-compilation for each platform.