Source : Free On-Line Dictionary of Computing

window system
     
        Software which allows a {workstation}'s screen 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}, and proprietary systems on
        the {Macintosh} and {NeXT}, {NeWS} on {Suns} and {RISC OS} on
        the {Archimedes}.  See also {WIMP}.