OAuth
The protocol that lets a site sign you in via Google/Facebook/GitHub without seeing your password.
"Sign in with Google" works like this: the site redirects you to Google, you approve ("yes, the site can read my email address"), Google sends back a signed token. The site now has proof of who you are without your Google password ever leaving Google.
OAuth 2.0 is what's used today. Often built on JWT tokens to represent the signed access.