OAuth
also: OAuth 2.0
The standard for letting an app act on your behalf on another service without ever seeing your password.
OAuth 2.0 is the protocol that lets one application access another service on a user’s behalf without handling the user’s password. The user authorizes the app, and it receives a scoped, revocable access token instead of credentials.
Worked example: “Sign in with Google” or letting an app post to your calendar uses OAuth — you approve specific scopes (read calendar, not delete email) and the app gets a token limited to those. Gotcha: OAuth is for AUTHORIZATION (what an app may do), not authentication (who you are) — adding identity on top is what OpenID Connect does; conflating the two, or requesting overly broad scopes, is the classic OAuth mistake.