An application programming interface (API) is a source code interface that an operating system or library provides to support requests for services to be made of it by computer programs.[1] Advanced programming interface is a near synonym with wider application that predates the current common usage. In the original term the concept is meant to represent any well defined interface between two separate programs. The main difference is that this older term does not inculcate a parent-child relationship and can therefore be applied to peer-to-peer situations more logically, e.g. internal kernel services which can present themselves as separate programs.