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A weight containing 480 g, or 112 pound troy and apothecaries' weight, or 437.5 g, 116 pound avoirdupois. The apothecary oz (used in the USP) contains 8 dram and is equivalent to 31.10349 g; the avoirdupois oz is equivalent to 28.35 g. [L. uncia, the twelfth part (of a pound or foot) hence also inch]
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1 a) a unit of troy weight equal to 1/12 troy pound or 31.103 grams
b) a unit of avoirdupois weight equal to 1/16 avoirdupois pound or 28.350 grams
2) FLUID OUNCE
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(oz) (ouns) [L. uncia] a measure of weight in both the avoirdupois and apothecaries' systems. The ounce avoirdupois is one sixteenth of a pound, or 437.5 grains (28.3495 g). The apothecaries' ounce, symbolMedical dictionary. 2011.