an accounting and authorization server which can be used to control access to a network or one host, too
an Authentication, Authorisation, and Access (AAA) server
a program which implements the central side of those protocols and generally runs on a central authority server box
a single database of users that one or more access servers query to authenticate a user
A server that is responsible for receiving user connection requests, authenticating the user, and then returning all of the configuration information necessary for the client to deliver the service to the user.
Remote Authentication Dial-In User Service (RADIUS) - a multi-user client-server security protocol used in computer networks to provide remote user authentication and accounting. The RADIUS software can read several kinds of password databases and use several kinds of authentication schemes. The protocol is described in RFC 2138, 2865, and 2866.
The acronym RADIUS stands for Remote Authentication Dial In User Service. It is both a protocol (defined in RFCs 2138, 2865, and 2866) and a server. The server is a software package that provides authentication and accounting services. The latest radius servers not only authenticate and charge users on dial-in connections; they provide complete control of access to networks. Thus, they are the appropriate tool for managing public-access wireless networks. Included in the software suite is the server program itself, radiusd, which responds to authentication and accounting requests, and accompanying programs to monitor the activity of the server and analyze the information it provides. This information includes user activity logs and more. One of the suppliers of this software is GNU; here is the GNU Radius Reference Manual.