Unit of weight in the avoirdupois system, the traditional European system of weight, which was incorporated into the British Imperial system and the U.S. system of weights and measures (seemeasurement). The ounce is equal to 1/16 lb (437.5 grains). In the troy and apothecaries' systems (two other traditional systems of weight), it is equal to 1/12 troy or apothecaries' lb (480 grains). The avoirdupois ounce is equal to 28.35 g, the troy ounce to 31.1 g. As a unit of volume, the fluid ounce is equal to 1/16 of a pint (29.57 ml) in the U.S. system, and to 1/20 of a pint (28.41 ml) in the British Imperial system. See alsogram, International System of Units, metric system, pound.