What is an API?

An API is a defined way for one piece of software to ask another for something: a payment taken, a message sent, a map drawn. It is how apps use services they did not build. When something says it "has an API", it means other software is allowed to talk to it.

For somebody building with AI, the practical meaning is narrower than the definition suggests. It usually comes up as a key, a long string you paste in that proves you are allowed to use a service, and which is billed to you. Treat it exactly like a password.

It also comes up as a reason things cost money unexpectedly. An API key that charges per request will keep charging as long as something keeps asking, including a mistake in a loop. Anything metered deserves a limit set on the provider's own dashboard, before you need it.