tachometer

noun

1. any of various instruments for measuring or indicating velocity or speed, as of a machine, a river, or the blood.
2. an instrument measuring revolutions per minute, as of an engine.

noun

1. any device for measuring speed, esp the rate of revolution of a shaft. Tachometers (rev counters) are often fitted to cars to indicate the number of revolutions per minute of the engine

n.1810, from Greek tachos “speed” + -meter. n.

1. An instrument used to measure the rotations per minute of a rotating shaft.