Definition from Wiktionary, the free dictionary
Alternative forms 
tachometer (plural tachometers)
- 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.
- (device for measuring velocity): speedometer
device for measuring rpm
device for measuring velocity — see speedometer