Tachometer n. : An instrument for measuring the velocity, or indicating changes in the velocity, of a moving body or substance.

Tachometer n. : An instrument for measuring the velocity of running water in a river or canal, consisting of a wheel with inclined vanes, which is turned by the current. The rotations of the wheel are recorded by clockwork.

Tachometer n. : An instrument for showing at any moment the speed of a revolving shaft, consisting of a delicate revolving conical pendulum which is driven by the shaft, and the action of which by change of speed moves a pointer which indicates the speed on a graduated dial.

Tachometer n. : An instrument for measuring the velocity of the blood; a haematachometer.

Example:
What causes Tachometer drift when the car is not running?

Taxonomy
tachometer, speed, vehicles, signal, wheel, RPM, Digital Tachometer, tractors, engine speed, sensors, measures, rotation, spindle, revolution, rotation speed, reference signal, free encyclopedia, gear.

Defination from the web:
Tachometer showing engine RPM (revolutions per minute), and a redline from 6000 and 7000 RPM. A tachometer (also called a revolution-counter, rev-counter, or RPM gauge) is an instrument that measures the rotation speed of a shaft or disk, ...