Application programming interface
An application programming interface (API) is the set of conventions by which a user application program written for a specific purpose communicates with software infrastructure such as the operating system, data base management services, web services, etc. APIs are specific to programming languages, although there may be multiple APIs to access the same service.
User applications, in this context, could serve either human or computer users. A web browser, word processor, or computer game is an application with a human interface. Alternatively, an electrical power grid manager, a missile guidance system or the control of a cardiac pacemaker is an application that serves a computer.
The term "application layer", in Computer networking reference models, refers to the services that support applications, not the applications themselves. APIs provide access to the top of the application layer.