: an apparatus used for determining concentration of dissolved substances (as hydrogen ions) in solution by color comparison with known standards
: an instrument, device, or set of charts (as for use in chemical analysis and medical diagnosis) for the determination and specification of colors by direct comparison with a standardized system of colors
Instrument for comparing something with a similar thing or with a standard measure, in particular to measure small displacements in mechanical devices. In astronomy, the blink comparator is used to examine photographic plates for signs of moving bodies. Machinists use comparators or visual gauges to centre or align work in machine tools.