Tachometer

1. (n.) An instrument for measuring the velocity, or indicating changes in the velocity, of a moving body or substance.
2. (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.
3. (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.
4. (n.) An instrument for measuring the velocity of the blood; a haematachometer.