An Acronym for Remote Authentication Dial In User Service.
RADIUS is an authentication, authorization and accounting protocol for terminals on a network, designed by a company called Livingston. A RADIUS server provides a central database for authentication that the remote end of your connection to an ISP can query to verify your credentials. This allows the account information to be maintained in a centralized resource and even be shared among ISPs if they share customers or resell one another's services. Terminals share a secret with the RADIUS server so that not just anyone can go checking passwords against it.
Implementations:
Check the FreeRADIUS project. They also provide pam_radius_auth, a PAM module that lets a Linux system authenticate users against a RADIUS server.
A Linux and WIndows RADIUS implementation commercial system to authenticate and account users against a RADIUS server.
See also:
5 pages link to RADIUS: