Avoirdupois (Fr. avoir du poids, to have weight; or, possibly, as it was formerly spelled averdupois, from the old Fr. verb arerer, to verify), a standard of weight, to which articles of merchandise sold by weight are referred, except the precious metals, gems, and medicines. The pound avoirdupois contains 7,000 grains; the pound troy contains 5,760. The ounces do not retain the same proportions, there being 16 to the pound avoirdupois, and 12 to the pound troy. The ounce avoirdupois is supposed to be the same as the Roman uncia, which, according to Dr. Arbuthnot, contained the same number of grains, viz;, 437 1/2; but it is very unlikely that these small weights have been preserved uniformly the same for so long a period. The old term avoirdupois is first met with in 1532, in some orders of Henry VIII.; and in 1588 a pound of this weight was deposited, by order of Queen Elizabeth, in the exchequer, as a standard. This, when examined in 1758 by the committee appointed by the government, was found to be 1 1/4 grain deficient in weight; and the troy weight was thereafter made the standard.
The standard grain, prescribed by act of parliament in the reign of George IV., is such that "a cubic inch of distilled water weighed in air by brass weights, at the temperature of 62° Fahrenheit's thermometer, the barometer being at 30 inches, is equal to 252.458 grains."