X
XCSP	External-call service provider. Subsystem that interoperates with external call protocols to provide services such as modem call setup and teardown.
XML	Extensible Markup Language. Standard that defines a syntax to let you create markup languages that specify information structures.
X-series recommendations	ITU-T standards that define data networks and open-system communications. Well-known X-series recommendations include X.25 for packet switching, X.400 for message-handling systems, and X.500 for directory services.