An API is the agreed-upon interface one system exposes so that others can use it without knowing how it works inside. It defines what you can ask for, what you must send, and what you get back. Every integration — a mobile app talking to a server, two companies’ systems exchanging data, a website calling a payment provider — runs over an API.
A good API is a product in its own right: stable, documented, versioned, and designed so that the systems built on top of it do not break when it evolves. Treating an API as an afterthought is how integrations become brittle and expensive to change later.
Related terms