Radius

http://www.webopedia.com/TERM/R/RADIUS.html

Short for Remote Authentication Dial-In User Service, an authentication and accounting system used by many Internet Service Providers (ISPs). When you dial in to the ISP you must enter your username and password. This information is passed to a RADIUS server, which checks that the information is correct, and then authorizes access to the ISP system. Though not an official standard, the RADIUS specification is maintained by a working group of the IETF.

http://cloudessa.com/products/cloudessa-radius-service/what-is-a-radius-server/

A RADIUS server is a network server implementing RADIUS and RADIUS-based authentication protocols, such as PAP, CHAP, MS-CHAP, PEAP, EAP-TTLS, EAP-TTLS, SIP Digest. Cloudessa provides virtual RADIUS servers as SaaS and as a Virtual Appliance. A RADIUS server implements RFC 2865 and RFC 2866 RADIUS authentication and accounting protocols, which are UDP-based protocols. During the RADIUS authentication phase a network client connects to a network access server (NAS) and provides authentication credentials. The NAS then uses the authentication credentials to issue a RADIUS authentication request to the RADIUS server. The RADIUS server and the NAS will then exchange RADIUS authentication messages. Once the authentication completes, the RADIUS server passes an “Accept” or “Reject” message to the NAS. The NAS will then permit or reject connection of the client to the network. Once the client is on the network the NAS will periodically send to the RADIUS server RADIUS accounting messages documenting client activity, such as the amount of data transferred to/from the client. When the client disconnects from the network, the NAS will send an accounting stop message to the RADIUS server. RADIUS attributes can be added to any RADIUS message and can be used to exchange additional information between the NAS and the RADIUS server. As an example, the NAS can inform the RADIUS server about the MAC address of the client. The RADIUS server can pass back to the NAS security policies that the NAS needs to enforce for a particular client. There is a set of standard RADIUS attributes specified in the RADIUS protocol specifications. In addition to that, many vendors define their own, vendor-specific attributes (VSAs) that can be used to control network equipment originating from a particular vendor.

http://searchsecurity.techtarget.com/definition/RADIUS

Remote Authentication Dial-In User Service (RADIUS) is a client/server protocol and software that enables remote access servers to communicate with a central server to authenticate dial-in users and authorize their access to the requested system or service.

Other references
 * https://msdn.microsoft.com/en-us/library/cc755248.aspx
 * https://technet.microsoft.com/it-it/library/cc726017(v=ws.10).aspx
 * https://www.juniper.net/techpubs/software/aaa_802/sbrc/sbrc70/sw-sbrc-admin/html/Concepts2.html
 * http://www.excitingip.com/475/what-is-a-radius-server/