SAML
An XML-based standard that lets an identity provider vouch for a user to an application, enabling single sign-on.
The application redirects the user to the identity provider, which authenticates them and returns a signed assertion confirming who they are and what groups they belong to. The application trusts the signature rather than handling passwords itself.
It remains the most widely supported enterprise SSO protocol. Configuration is where the mistakes happen, particularly around signing and validating the response as well as the assertion.
Where this comes up
SAML sits inside our identity and access work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.