Short for Terminal Access Controller Access Control System, an authentication protocol that was commonly used in UNIX networks. TACACS allows a remote access server to communicate with an authentication server in order to determine if the user has access to the network.
TACACS is now somewhat dated and is not used as frequently as it once was. A later version of TACACS was called XTACACS (Extended). These two versions have generally been replaced by TACACS+ and RADIUS in newer or updated networks. TACACS+ is a completely new protocol and is therefore not compatible with TACACS or XTACACS.
TACACS is detailed in RFC 1492.