API
The set of requests a program accepts from other programs, and what it answers.
An application programming interface is the documented set of operations a component exposes -- for a web back end, its HTTP methods, paths, request and response bodies, and error codes.
Examples
The greenhouse back end's API includes
GET /devices/gh-a1/readingsfor history andPOST /devicesto register a node, each with defined request bodies and status codes.
See also