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.

   (http://x.org/).

   See also Andrew project, PEX, VNC, XFree86.

   Usenet newsgroups: news:comp.windows.x, news:comp.x,
   news:comp.windows.x.apps, news:comp.windows.x.intrinsics,
   news:comp.windows.x.announce, news:comp.sources.x,
   news:comp.windows.x.motif, news:comp.windows.x.pex.

   (1999-04-02)