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

pixmap

   (Contraction of "pixel map").  A 3 dimensional array of bits
   corresponding to a 2 dimensional array of pixels.  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].