OAuth

OAuth is the standard that lets one application act on your behalf in another without ever seeing your password. It is what happens behind "Sign in with Google" and behind granting an application access to your calendar or repositories.

The exchange is a delegated, scoped, revocable token: the application receives permission to do specific things, you can withdraw it later, and your credentials never leave the provider. That last property is the whole point — an integration that asks for your actual password is asking for something it cannot be trusted with.

It is frequently confused with authentication. OAuth grants access to resources; OpenID Connect is the layer on top of it that actually establishes identity, and is what a "sign in with" button should be using.