API

An API, or Application Programming Interface, is a contract that lets one piece of software use another without knowing its internal details. On the web, an API usually means a set of URLs a program can call to read or change data, often exchanging JSON over HTTP. The API defines what requests are valid, what parameters they take, and what responses to expect. Good APIs are predictable and documented, so developers can build on them confidently. The idea is broader than the web, though: any well-defined boundary between components — a library’s functions, an operating system’s calls — is an API.