X.25 (interface card)
A recommendation published by the Comite Consultatif International de Telegraphique et Telephonique (CCITT) international communications standards organization, X.25 defines the connection between a terminal and a packet-switching network. An X.25 network is a type of packet-switching network that routes units of information (packets) as specified by X.25 and is used in public data communications networks. See also packet.
X.400 system
A messaging system that is compliant with the X.400 standards developed under the Comite Consultatif International de Telegraphique et Telephonique (CCITT) and the International Standards Organization (ISO).
X Windows
A graphical windowing system for UNIX computers.