The Free On-line Dictionary of Computing (30 December 2018):
X Window System
A specification for
device-independent windowing operations on bitmap display
devices, developed initially by MIT's Project Athena and
now a de facto standard supported by the X Consortium. X
was named after an earlier window system called "W". It is a
window system called "X", not a system called "X Windows".
X uses a client-server protocol, the X protocol. The
server is the computer or X terminal with the screen,
keyboard, mouse and server program and the clients are
application programs. Clients may run on the same computer
as the server or on a different computer, communicating over
Ethernet via TCP/IP protocols. This is confusing because
X clients often run on what people usually think of as their
server (e.g. a file server) but in X, it is the screen and
keyboard etc. which is being "served out" to the applications.
X is used on many Unix systems. It has also been described
as over-sized, over-featured, over-engineered and incredibly
over-complicated. X11R6 (version 11, release 6) was released
in May 1994.
See also Andrew project, PEX, VNC, XFree86.
Usenet newsgroups: news:comp.windows.x, news:comp.x,