ounce

   1. (oz or oz av)

   a traditional unit of weight. The avoirdupois ounce, the unit commonly used in the United States, is 1/16 pound or about 28.3495 grams.

   The avoirdupois ounce also equals 175/192 = about 0.911 457 troy ounce or 437.5 grains. The word ounce is from the Latin uncia, meaning a 1/12 part, because the Roman pound was divided into 12 ounces. The word "inch," meaning 1/12 foot, has the same root. The symbol oz is from the old Italian word onza (now spelled oncia) for an ounce. See avoirdupois weights for additional information.

   2. (oz, oz t, toz, or oz ap)

   a second traditional unit of mass or weight. The troy ounce, traditionally used in pharmacy and jewelry, is 1/12 troy pound, 480 grains, or about 31.1035 grams. Thus the troy ounce equals 192/175 = 1.09714 avoirdupois ounces. This unit is the traditional measure for gold and other precious metals; in particular, the prices of gold and silver quoted in financial markets are the prices per troy ounce. The troy ounce is divided into 20 pennyweight or into 8 troy drams². See troy weights for additional information. The troy ounce is sometimes abbreviated oz t or toz to distinguish it from the more common avoirdupois ounce; in traditional pharmacy it was abbreviated oz ap.

   3. (oz or fl oz)

   a traditional unit of liquid volume, also called the fluid ounce (fl oz).

   4. an old term for a 1/12 part, the English equivalent of the Latin uncia (see def.¹ above). In medieval times, the word was used sometimes for a unit of distance equal to 1/12 yard or 3 inches. It was also used for a unit of time equal to 1/12 moment, or 7.5 seconds. In some settings, an ounce of time was divided into exactly 47 atoms.