API Key
also secret key
A secret string that identifies and authenticates your application to an API, usually sent in a header with each request.
Analogy
Like a membership card number that proves who you are at the door.
Why it matters
API keys authenticate your HTTP calls and must be stored as secrets, never hardcoded, when wiring up integrations.
In practice
Sending Authorization: Bearer sk-... in a header to call an API.