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

grey-scale

    (US "gray-scale") 1. Composed of (discrete) shades
   of grey.  If the pixels of a grey-scale image have N
   bits, 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)