Avoirdupois (/ˌævərdəˈpɔɪz, ˌævwɑːrdjuːˈpwɑː/; abbreviated avdp.) is a measurement system of weights that uses pounds and ounces as units. It was first commonly used in the 13th century AD and was updated in 1959.

In 1959, by international agreement among countries that used the pound as a unit of mass, the International Avoirdupois Pound was fixed at the modern definition of exactly 0.45359237 kilograms.. It remains the everyday system of weights used in the United States, and is still used, in varying degrees, in everyday life in the United Kingdom, Canada, Australia^{[citation needed]}, and some other former British colonies, despite their official adoption of the metric system.

The avoirdupois weight system's general attributes were originally developed for the international wool trade in the Late Middle Ages, when trade was in recovery. It was historically based on a physical standardized pound or "prototype weight" that could be divided into 16 ounces. There were a number of competing measures of mass, and the fact that the avoirdupois pound had three even numbers as divisors (half and half and half again) may have been a cause of much of its popularity, so that the system won out over systems with 12 or 10 or 15 subdivisions. The use of this unofficial system gradually stabilized and evolved, with only slight changes in the reference standard or in the prototype's actual mass.

Over time, the desire not to use too many different systems of measurement allowed the establishment of "value relationships", with other commodities metered and sold by weight measurements such as bulk goods (grains, ores, flax) and smelted metals, so the avoirdupois system gradually became an accepted standard through much of Europe.

In England, Henry VII authorized its use as a standard, and Queen Elizabeth I acted three times to enforce a common standard, thus establishing what became the Imperial system of weights and measures. Late in the 19th century various governments acted to redefine their base standards on a scientific basis and establish ratios between local avoirdupois measurements and international SI metric system standards. The legal actions of these various governments were independently conceived, and so did not always pick the same ratios to metric units for each avoirdupois unit. The result of this was, after these standardisations, measurements of the same name often had marginally different recognised values in different regions (although the pound generally remained very similar). In the modern day, this is evident in the small difference between United States customary and British Imperial pounds.

An alternative system of mass, the troy system, also denominated in pounds and ounces, is generally used for precious materials.

The word avoirdupois is from Anglo-Norman French aveir de peis (later avoir du pois), literally "goods of weight" (Old French aveir, as verb meaning "to have" and as noun meaning "property, goods", comes from the Latin habere, "to have, to hold, to possess something"; de = "from"/"of", cf. Latin; peis = "weight", from Latin pēnsum.) This term originally referred to a class of merchandise: aveir de peis, "goods of weight", things that were sold in bulk and were weighed on large steelyards or balances.

Only later did the term become identified with a particular system of units used to weigh such merchandise. Inconsistent orthography throughout history has left many variants of the term, such as haberty-poie and haber de peyse. (The Norman peis became the Parisian pois. In the 17th century de was replaced with du.)