A documented rant has made the rounds at https://sso.tax , which lists all vendors and their pricing of SSO.
Single sign-on (SSO) is an authentication scheme that allows a user to log in with a single ID to any of several related, yet independent, software systems.
SCIM: System for Cross-domain Identity Management
System for Cross-domain Identity Management (SCIM) is a standard for automating the exchange of user identity information between identity domains, or IT systems.
SAML: Security Assertion Markup Language
Security Assertion Markup Language (SAML, pronounced SAM-el, /ˈsæməl/)[1] is an open standard for exchanging authentication and authorization data between parties, in particular, between an identity provider and a service provider.
https://en.wikipedia.org/wiki/Single_sign-on
https://en.wikipedia.org/wiki/System_for_Cross-domain_Identi...
https://en.wikipedia.org/wiki/Security_Assertion_Markup_Lang...
(if you work at SpaceX, SSO might also mean Single Stage to Orbit, which is lots more exciting - but since Elon banned acronyms maybe it's not used)