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tachometer ta·chom·e·ter (tā-kŏm'ĭ-tər, tə-)
An instrument used to measure the rotations per minute of a rotating shaft.
device for indicating the angular (rotary) speed of a rotating shaft. The term is usually restricted to mechanical or electrical instruments that indicate instantaneous values of speed in revolutions per minute, rather than devices that count the number of revolutions in a measured time interval and indicate only average values for the interval.