Source : Free On-Line Dictionary of Computing

pixmap
     
        (Contraction of "pixel map").  A 3 dimensional {array} of bits
        corresponding to a 2 dimensional array of {pixel}s.  It is
        used, for example, in the {X Window System} to describe a
        memory region where graphics can be drawn without affecting
        the screen.  Typically this is used for the efficient handling
        of {expose} events, {icon} images or for animation.  Compare
        {bitmap}.
     
        [Xlib Guide].