late 14c., standard system of weights for gems and precious metals, from Troyes, city in France (Roman (Civitas) Tricassium, capital of the Tricasses, a Celtic people whose name was said to mean "those with three tresses"), former site of an important fair at which this weight is said to have been used. Many medieval towns had their own standard weights. The pound troy contains 5,760 grains and is divided into 12 ounces.

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Definitions of troy from WordNet
troy (n.)
a system of weights used for precious metals and gemstones; based on a 12-ounce pound and an ounce of 480 grains;
Synonyms: troy weight
Troy (n.)
an ancient city in Asia Minor that was the site of the Trojan War;
Synonyms: Ilion / Ilium