n. 1 a a unit of weight of one-sixteenth of a pound avoirdupois (approx. 28 grams). °Abbr.: oz. b a unit of one-twelfth of a pound troy or apothecaries' measure, equal to 480 grains (approx. 31 grams). 2 a small quantity. fluid ounce Brit. 1 a unit of capacity equal to one-twentieth of a pint (approx. 0.028 litre). 2 US a unit of capacity equal to one-sixteenth of a pint (approx. 0.034 litre).
[ ME & OF unce f. L uncia twelfth part of pound or foot: cf. INCH(1) ]