Source : Webster's Revised Unabridged Dictionary (1913)

Tachometer \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.

Source : WordNet®

tachometer
     n : measuring instrument for indicating speed of rotation [syn:
         {tach}]