What is Remote Authentication Dial-In User Service (RADIUS)? A protocol that sets up a point-to-point connection between two computer systems over an Internet Protocol (IP) network. As part of AAA, it provides the ability of a system to collect statistics on networks or users for auditing and billing purposes. This enables the tracking of systems usage, start and stop times of resources, and number of packets, as well as other metrics that identify what was used and for how long. A client/server protocol that provides authentication, authorization, and accounting for a remote dial-in system. A tunneling protocol that encapsulates packets inside Internet Protocol (IP) tunnels.

This is the best description of a Remote Authentication Dial-In User Service (RADIUS). Remote Authentication Dial-In User Service (RADIUS) is a networking protocol that provides centralized authentication, authorization and accounting. These are provided to users of a network service, enabling a remote dial-in system.