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An ounce (symbol: oz) is a unit of mass in the imperial and US customary systems of measurement. The avoirdupois ounce, which is the most commonly used form, is defined as exactly 28.349523125 grams. There are 16 ounces in one pound, making the ounce one of the fundamental units of the imperial weight system used daily in the United States, United Kingdom, and other countries that retain imperial measurements. The term "ounce" can refer to several different units depending on context. The avoirdupois ounce (28.35 g) is used for most everyday weight measurements including food, body weight, and commerce. The troy ounce (31.1035 g) is used exclusively for precious metals such as gold, silver, and platinum. The fluid ounce is a separate unit of volume, not mass, though it shares the name. For cooking and food labeling, the avoirdupois ounce is the standard in the United States. Nutritional information on US food packaging is commonly given per ounce or per 28g serving, reflecting the near-equivalence of one ounce to 28 grams.

The ounce has ancient roots, tracing back to the Roman uncia, which was one-twelfth of the Roman pound (libra). The word "ounce" derives directly from uncia, as does the word "inch" — both representing one-twelfth of their respective base units in the Roman system. This dual origin explains why the abbreviation for ounce is "oz," coming from the Italian "onza." During the medieval period, various ounce standards coexisted in Europe. The troy system, originating in the French city of Troyes (a major medieval trading center), used 12 ounces per pound and became the standard for precious metals and medicines. The avoirdupois system, developed for general commerce, established 16 ounces per pound and became dominant for everyday goods. In 1959, the international yard and pound agreement standardized the avoirdupois ounce at exactly 28.349523125 grams across the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa. This international standard remains in use today, ensuring consistency in trade and manufacturing across countries that use the imperial system.

The ounce remains the primary unit of small mass measurement in the United States for everyday applications including food portions, cooking ingredients, and packaged goods. US recipes, nutritional labels, and food packaging routinely express quantities in ounces. Body weight is typically expressed in pounds and ounces for newborns and small weight differences. In international trade, ounces appear frequently on product labels sold in the US market, often alongside metric equivalents. The troy ounce is universally used in precious metals markets worldwide, regardless of whether a country uses metric or imperial measurements. Jewelers and bullion dealers globally quote gold and silver prices per troy ounce.