RADIUS, which stands for Remote Authentication Dial-In User Service, is a networking protocol that provides centralized authentication, authorization, and accounting for users who connect to a network. It is commonly used by organizations to manage access to their systems and services, ensuring that only authorized users can connect. The protocol operates by allowing a client device, such as a router or switch, to communicate with a RADIUS server. When a user attempts to access the network, the client sends the user's credentials to the server, which verifies them and grants or denies access based on predefined policies.