The Free On-line Dictionary of Computing (30 December 2018):

window system
windowing
windowing system

    Software which allows a computer's
   display to be divided into rectangular areas which act like a
   separate input/output devices under the control of different
   application programs.  This gives the user the ability to see
   the output of several processes at once and to choose which one
   will receive input by selecting its window, usually by pointing at
   it with a mouse.

   Examples are the X Window System, proprietary systems on the
   Macintosh and NeXT, NeWS on Suns, RISC OS on the
   Archimedes and Microsoft Windows.  See also WIMP.

   (2015-03-07)