\Ta*chom"e*ter\, n. [Gr. ? swiftness, speed (fr.
tachy`s quick) + -meter: cf. F. tachom[`e]tre.]
An instrument for measuring the velocity, or indicating
changes in the velocity, of a moving body or substance.
Specifically:
(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.
(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.
(c) (Physiol.) An instrument for measuring the velocity of
    the blood; a h[ae]matachometer.