What word does your mother always say?
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.