Source : Free On-Line Dictionary of Computing

bit plane
     
         (Or "bitplane") The memory in a graphic display
        device which holds a complete one-bit-per-{pixel} image.
        Several bit planes may be used in conjunction to give more
        bits per pixel or to overlay several images or mask one with
        another.
     
        "Bit plane" may be used as a synonym for "{bitmap}", though
        the latter suggests the data itself rather than the memory and
        also suggests a graphics file format.
     
        (1997-03-16)