Ounce derives from Latin uncia. The ounce is part of the weights system, derived from the Roman monetary system. The Romans used bronze bars of varying weights as currency. An aes grave ("heavy bronze") weighed 1 pound. One twelfth of an aes grave was called an uncia, or in English an "ounce". The ounce is defined as exactly 28.349523125 g. But ounce isn't like troy ounce.
The name "troy ounce" is derived from Troyes, a trade market in France. Merchants came from all over the world to buy and sell goods, so a standardized weight system would have made doing business much easier.
Troy ounce is used in the pricing of metals such as gold, platinum and silver. One troy ounce is defined as exactly 31.1034768 g, which may be used to denote the value of a precious metal. When the price of gold is said to be 1200 EUR/ounce, the ounce being referred to is a troy ounce, not a standard ounce.
