RADIUS
A network-access protocol from the 1990s that centralizes authentication, authorization, and accounting for who gets onto the network — VPNs, Wi-Fi, and switches all lean on it.
RADIUS is the gatekeeper of network access rather than application access — the layer below SAML or OAuth, deciding whether a device even gets onto the network before any app-level login happens. EAP methods define how the actual credential exchange works inside a RADIUS conversation, from simple passwords to certificate-based authentication.