Source : Free On-Line Dictionary of Computing

grey-scale
     
         (US "gray-scale") 1. Composed of (discrete) shades
        of grey.  If the {pixels} of a grey-scale {image} have N
        {bit}s, they may take values from zero, representing black up
        to 2^N-1, representing white with intermediate values
        representing increasingly light shades of grey.  If N=1 the
        image is not called grey-scale but could be called
        {monochrome}.
     
        2. A range of acurately known shades of grey printed out for
        use in calibrating those shades on a display or printer.
     
        (1995-03-17)