What is Ounce?

Legal Definition
An ounce (abbreviated oz; apothecary symbol: ) is a unit of mass used in most British derived customary systems of measurement. It is most pervasive in the retail sale of groceries in the United States, but is also used in many other matters of domestic and international trade. Similar customary uses include recipes in cookbooks and sales of bulk dry goods.

Whilst various definitions have been used throughout history, two remain in common use: the avoirdupois ounce equal to approximately 28.3 grams and the troy ounce of about 31.1 grams. The avoirdupois ounce is widely used as part of the United States customary and British imperial systems, but the troy ounce is now only commonly used for the mass of precious metals such as gold, silver, platinum, palladium, rhodium, etc..
-- Wikipedia
Legal Definition
The name of a weight. An ounce avoirdupois weight is the sixteenth part of a pound; an ounce troy weight is the twelfth part of a pound. Vide Weights.
-- Bouviers Law Dictionary
Legal Definition
The twelfth part; the twelfth part of a pound troy or the sixteenth part of a pound avoirdupois.
-- Black's Law Dictionary