Radius

Short for Remote Authentication Dial-In User Service, an authentication and accounting system used by many Internet Service Providers (ISPs).

When you dial in to the ISP you must enter your username and password. This information is passed to a RADIUS server, which checks that the information is correct, and then authorizes access to the ISP system.

Though not an official standard, the RADIUS specification is maintained by a working group of the IETF.