Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

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

   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.

Source : WordNet®

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}]