Service Provider (SP)
The application a user is trying to reach — it outsources authentication to an identity provider and trusts the assertion or token that comes back instead of checking a password itself.
aka: SP, relying party
“Service provider” is SAML’s name for the role; OIDC calls the same role a “relying party,” and OAuth calls it a “client.” Whatever the protocol names it, the pattern is identical: the app that needs to know who’s using it delegates the hard, security-sensitive part — verifying a credential — to a system built specifically for that job.