tachometer
noun
- measuring device used to measure rotational speed
Wiktionary
Pronunciation: /təˈkɒmətə(ɹ)/
noun
Etymology: From tacho- + -meter.
- A device for measuring the revolutions per minute (RPMs) of a revolving shaft, as with the driveshaft of an automobile.
- A device for measuring or indicating velocity or speed, as of blood, a river, a machine, etc.