SSO
Acronymsingle-sign-on, single sign onDefinition
Single sign-on — signing in once, with one identity provider, and being signed in to every application that trusts that provider; the application never sees a password, only an assertion from the provider that this person is who they say
The goal, not the mechanism: SAML and OpenID Connect (on OAuth) are the two protocols that deliver it, the first in the enterprise, the second on the consumer web. The application delegates the hard part — passwords, second factors, lockouts, recovery — to the provider, and gets an identity it did not have to verify. "Sign in with GitHub" on this site is SSO with GitHub as the provider, beside the magic link and the OTPs that are the site's own.
One password, kept in one place, is the security argument as much as the convenience one: an application that never stores a credential cannot leak it, and an account disabled at the provider is disabled everywhere at once.