The Collaborative International Dictionary of English v.0.48:

Carat \Car"at\ (k[a^]r"[a^]t), n. [F. carat (cf. It. carato,
   OPg. quirate, Pg. & Sp. quilate), Ar. q[imac]r[=a]t bean or
   pea shell, a weight of four grains, a carat, fr. Gr.
   kera`tion a little horn, the fruit of the carob tree, a
   weight, a carat. See Horn.]
   1. The weight by which precious stones and pearls are
      weighed.
      [1913 Webster]

   Note: The carat equals three and one fifth grains Troy, and
         is divided into four grains, sometimes called carat
         grains. Diamonds and other precious stones are
         estimated by carats and fractions of carats, and
         pearls, usually, by carat grains. --Tiffany.
         [1913 Webster]

   2. A twenty-fourth part; -- a term used in estimating the
      proportionate fineness of gold.
      [1913 Webster]

   Note: A mass of metal is said to be so many carats fine,
         according to the number of twenty-fourths of pure gold
         which it contains; as, 22 carats fine (goldsmith's
         standard) = 22 parts of gold, 1 of copper, and 1 of
         silver.
         [1913 Webster]

WordNet (r) 3.0 (2006):

carat
    n 1: a unit of weight for precious stones = 200 mg
    2: the unit of measurement for the proportion of gold in an
       alloy; 18-karat gold is 75% gold; 24-karat gold is pure gold
       [syn: karat, carat, kt]

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

CARAT, weights. A carat is a weight equal to three and one-sixth grains, in
diamonds, and the like. Jac. L. Dict. See Weight.