The Collaborative International Dictionary of English v.0.48:

Troy \Troy\, n.
   Troy weight.
   [1913 Webster]

   Troy weight, the weight which gold and silver, jewels, and
      the like, are weighed. It was so named from Troyes, in
      France, where it was first adopted in Europe. The troy
      ounce is supposed to have been brought from Cairo during
      the crusades. In this weight the pound is divided into 12
      ounces, the ounce into 20 pennyweights, and the
      pennyweight into 24 grains; hence, the troy ounce contains
      480 grains, and the troy pound contains 5760 grains. The
      avoirdupois pound contains 7000 troy grains; so that 175
      pounds troy equal 144 pounds avoirdupois, or 1 pound troy
      = 0.82286 of a pound avoirdupois, and 1 ounce troy =
      117/175 or 1.09714 ounce avoirdupois. Troy weight when
      divided, the pound into 12 ounces, the ounce into 8 drams,
      the dram into 3 scruples, and the scruple into 20 grains,
      is called apothecaries' weight, used in weighing
      medicines, etc. In the standard weights of the United
      States, the troy ounce is divided decimally down to the
      1/10000 part.
      [1913 Webster]

WordNet (r) 3.0 (2006):

troy weight
    n 1: a system of weights used for precious metals and gemstones;
         based on a 12-ounce pound and an ounce of 480 grains [syn:
         troy, troy weight]

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

TROY WEIGHT. A weight less ponderous than the avoirdupois weight, in the
proportion of seven thousand, for the latter, to five thousand seven hundred
and sixty, to the former. Dane's Ab. Index, h.t. Vide Weights.