SAML 2.0
An XML-based standard for exchanging authentication and authorization assertions between an identity provider and a service provider, best known for enterprise web SSO.
aka: SAML, Security Assertion Markup Language
SAML establishes trust through exchanged metadata, moves signed (and optionally encrypted) XML assertions between IdP and SP, and predates OAuth and OIDC by a decade — which is why it still runs enterprise SSO for older or highly regulated back-office software long after consumer apps moved on to newer protocols.