Glossary
Identity Provider (IdP)
An identity provider is the system other services defer to for the question 'is this really who it claims to be.' Rather than every service implementing its own login and credential storage, they trust assertions from a shared identity provider — a design that centralizes both the convenience and the risk: get the identity provider wrong, and every service that trusts it inherits the mistake.
This is why an identity provider's own security posture — how it protects its signing keys, how it handles compromised accounts, how quickly it can revoke a session across every service that trusts it — matters disproportionately to the rest of a system's security. It is functioning, in effect, as a trust root for identity specifically.