Definition of Avoirdupois weight:
Imperial unit for all goods sold by weight except precious metals (sold by Troy ounces). It is based on a pound of 16 ounces, each ounce being equal to 16 drams. Introduced in the 14th century UK, it was legally instituted in 1824, and replaced by the Metric system in 1990. From the French avoir du poids, goods of weight.
Meaning of Avoirdupois weight & Avoirdupois weight Definition