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.