API

A defined set of endpoints and rules that lets one piece of software request data or functionality from another, without needing to know how it's implemented internally.

Example: Calling GET /users/42 to fetch a user's profile without knowing what database sits behind it.

In practice: Almost every other term in this glossary's 'APIs & Systems' category is a refinement of some aspect of API design — REST and GraphQL are styles, contracts and schemas are what an API promises.