RADIUS is an acronym for Remote Access Dial In User Service. RADIUS was part of an AAA solution delivered by Livingston Enterprises to Merit Network in 1991. Merit Network is a non-profit Internet provider, which required a creative way to manage dial-in access to various Points-Of-Presence (POPs) across it's network.
The solution supplied by Livingston Enterprises had a central user store used for authentication. This could be used by numerous RAS (dial-in) servers. Authorization and accounting could also be done whereby AAA was satisfied. Another key aspect of the Livingston solution included proxying to allow scaling.
The RADIUS protocol was then subsequently published in 1997 as RFCs, some changes applied, and today we have RFC2865, which ...