What is Avoirdupois?

Legal Definition
The avoirdupois system (English pronunciation: /ˌævərdəˈpɔɪz/; abbreviated avdp) is a measurement system of weights which uses pounds and ounces as units. It was first commonly used in the 13th century and was updated in 1959.

In 1959, by international agreement, the definitions of the pound and ounce became standardized in countries which use the pound as a unit of mass. The International Avoirdupois Pound was then created. It is the everyday system of weights used in the United States. It is still used, in varying degrees, in everyday life in the United Kingdom, Canada, 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 multiples (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 ratio-metric equations to SI metric system standards. They did not always pick the same equivalencies, though the pound remained very similar; these independent legal actions led to small differences in certain quantities, such as the American and Imperial pounds.

An alternative system of mass, the troy system, is generally used for precious materials. The modern definition of the avoirdupois pound (1 lb) is exactly 0.45359237 kilograms.
-- Wikipedia
Legal Definition
The name of a system of weights (sixteen ounces to the pound) used in weighing articles other than medicines, metals, and precious stones.
-- Black's Law Dictionary