XBM

from The Free On-line Dictionary of Computing (8 July 2008)

xbm

   <filename extension> X bit map.  The {filename extension} for
   files containing {bitmaps} for use in the {X Window System}.

   These are actually {include files} which can be #include'd
   into {C} program source to define images as initialised data.

   (1995-03-14)