1. A unit of mass in avoirdupois measure equal to ¹/₁₆ pound or to approximately 0.0283495 kilogram. Abbreviated oz.

2. A unit of mass in either troy or apothecaries' measure equal to 480 grains or exactly 0.0311034768 kilogram. Also known as apothecaries' ounce or troy ounce (abbreviations are oz ap and oz t in the United States, and oz apoth and oz tr in the United Kingdom).