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

WordNet (r) 3.0 (2006):

hydrometer
    n 1: a measuring instrument for determining the specific gravity
         of a liquid or solid [syn: hydrometer, gravimeter]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

HYDROMETER. An instrument for measuring the density of fluids; being
immersed in fluids, as in water, brine, beer, brandy, &c., it determines the
proportion of their densities, or their specific gravities, and thence their
qualities.
     2. By, the Act of Congress of January 12, 1825, 3 Story's' Laws U. S.
1976, the secretary of the treasury is authorized, under the direction of
the president of the United States, to adopt and substitute such hydrometer
as he may deem best calculated to promote the public interest, in lieu of
that now prescribed by law, for the purpose of ascertaining the proof of
liquors; and that after such adoption and substitution, the duties imposed
by law upon distilled spirits shall be levied, collected and paid, according
to the proof ascertained by any hydrometer so substituted and adopted.