API key

Many services authorize access with an API key: a long secret string an app includes with each request to prove it is allowed. It is the simplest way to let two systems talk, and the reason credentials have to be kept in an encrypted vault rather than pasted in plain sight.

An API key is powerful and blunt: whoever holds it can act as you against that service. That is why keys are stored encrypted, scoped where possible, and rotated if ever exposed. OAuth is the more granular, revocable alternative.