An instrument that measures colour directly as three numbers through filters that imitate the eye.
It uses three or more filtered sensors approximating the CIE colour-matching functions to report tristimulus values. Display calibrators are usually colorimeters.
Because it records only three numbers, a colorimeter cannot tell two metameric samples apart, and its accuracy depends on how well its filters match the standard observer for the light being measured.
A screen-calibration puck is a colorimeter tuned for display spectra.
Cheaper and faster than a spectrophotometer, and good for repeated measurements of similar things.