The Collaborative International Dictionary of English v.0.48:

Hydrometer \Hy*drom"e*ter\, n. [Hydro-, 1 + -meter: cf. F.
   hydrom[`e]tre.]
   1. (Physics) An instrument for determining the specific
      gravities of liquids, and thence the strength spirituous
      liquors, saline solutions, etc.
      [1913 Webster]

   Note: It is usually made of glass with a graduated stem, and
         indicates the specific gravity of a liquid by the depth
         to which it sinks in it, the zero of the scale marking
         the depth to which it sinks in pure water. Extra
         weights are sometimes used to adapt the scale to
         liquids of different densities.
         [1913 Webster]

   2. An instrument, variously constructed, used for measuring
      the velocity or discharge of water, as in rivers, from
      reservoirs, etc., and called by various specific names
      according to its construction or use, as tachometer,
      rheometer, hydrometer, pendulum, etc.; a current
      gauge. Hydrometric

The Collaborative International Dictionary of English v.0.48:

Tachometer \Ta*chom"e*ter\, n. [Gr. ta`chos 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:
   [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]

WordNet (r) 3.0 (2006):

tachometer
    n 1: measuring instrument for indicating speed of rotation [syn:
         tachometer, tach]