Put those together and you have a promise between two programs.
API
Also called Web API, Endpoint
The set of requests a program agrees to answer, and the answers it promises to give. It's a contract between two pieces of software, so that neither side has to read the other's code.
Two ways this goes wrong with agents. They invent endpoints (the separate addresses an API answers at) and fields that sound right but don't exist, so point them at real docs. And they rename a field on your server without knowing who else reads it, and nothing fails until a client shows nonsense.
“Don't change the shape of GET /api/orders, because other clients use it. Only add fields. Update openapi.yaml in the same commit, and add a contract test that fails if a documented field disappears or changes type.”