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A contract that lets one program ask another program for data or actions.
An API defines what you can call, what inputs are accepted, and what comes back. When your app asks Stripe to create a checkout session or asks your backend for a user profile, it is using an API. Good APIs are predictable: clear endpoints, clear request shape, clear response shape, and useful errors.
Plainly
Think of API as a named building block in a big LEGO app. A contract that lets one program ask another program for data or actions.
In practice
Use it when you are mapping how the app is structured or explaining a feature to a teammate or agent. In practice, define the owner, input, output, and failure mode before you rely on it.