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.