API...
An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software.1(https://en.wikipedia.org/wiki/API#cite_note-1) A document or standard that describes how to build or use such a connection or interface is called an API specification. A computer system that meets this standard is said to implement or expose an API. The term API may refer either to the specification or to the implementation.
A diagram from 1978 proposing the expansion of the idea of the API to become a general programming interface, beyond application programs alone.5(https://en.wikipedia.org/wiki/API#cite_note-NBS1981-5)