Diameter is an authentication, authorization and accounting protocol

AAA protocol

In computer security, AAA commonly stands for authentication, authorization and accounting.- Authentication :Authentication refers to the process where an entity's identity is authenticated, typically by providing evidence that it holds a specific digital identity such as an identifier and the...

 for computer networks, and an alternative to RADIUS

RADIUS

Remote Authentication Dial In User Service is a networking protocol that provides centralized Authentication, Authorization, and Accounting management for computers to connect and use a network service...

.

Diameter Applications extend the base protocol by adding new commands and/or attributes, such as those for use of the Extensible Authentication Protocol

Extensible Authentication Protocol

Extensible Authentication Protocol, or EAP, is an authentication framework frequently used in wireless networks and Point-to-Point connections. It is defined in RFC 3748, which made RFC 2284 obsolete, and was updated by RFC 5247....

 (EAP).
The name is a pun on the RADIUS

RADIUS

Remote Authentication Dial In User Service is a networking protocol that provides centralized Authentication, Authorization, and Accounting management for computers to connect and use a network service...

 protocol, which is the predecessor (a diameter is twice the radius). Diameter is not directly backwards compatible

Backward compatibility

In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...

, but provides an upgrade path for RADIUS.