tachometer

n. device that measures the revolutions per minute of a revolving shaft

A tachometer is an instrument that measures the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the rate of revolutions per minute on a calibrated analog dial, but digital displays are increasingly common. The term comes from Greek Ταχος, tachos, "speed", and metron, "to measure".
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Ta·chom·e·ter n. An instrument used to measure the rotations per minute of a rotating shaft. [more]Tachometer - Community and Resources

Noun
1. measuring instrument for indicating speed of rotation
(synonym) tach
(hypernym) measuring instrument, measuring system, measuring device
(hyponym) tachograph

n. tachometer, device that measures the revolutions per minute of a revolving shaft