a system of weights in use for precious metals and gems (formerly also for bread, grain, etc.): 24 grains = 1 pennyweight (1.555 grams); 20 pennyweights = 1 ounce (31.103 grams); 12 ounces = 1 pound (0.373 kilogram). The grain, ounce, and pound are the same as in apothecaries' weight, the grain alone being the same as in avoirdupois weight. The troy pound is no longer a standard weight in Great Britain.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.