API (Application Programming Interface)

An API (Application Programming Interface) is a defined way for one piece of software to use another. It specifies the requests a program can make, the data it must send and what it gets back, while hiding how the other side works internally.

APIs are everywhere: web APIs let apps fetch data over HTTP (for example weather, payments or maps), operating system APIs let programs use files and devices, and library APIs expose functions to developers. A good API is documented, consistent and stable so others can build on it.