n. [ Gr. ta`chos swiftness, speed (fr. tachy`s quick) + -meter: cf. F. tachomètre. ] An instrument for measuring the velocity, or indicating changes in the velocity, of a moving body or substance. Specifically: -- [ 1913 Webster ]

(a) 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. [ 1913 Webster ]

(b) 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. [ 1913 Webster ]

(c) (Physiol.) An instrument for measuring the velocity of the blood; a haematachometer. [ 1913 Webster ]