The avoirdupois ( /ˌævərdəˈpɔɪz/; French pronunciation: [avwaʁdypwɑ]) system (abbreviation: avdp) is a system of weights (or, properly, mass) based on a pound of 16 ounces. It is the everyday system of weight used in the United States and is still widely used to varying degrees in everyday life in the United Kingdom, Canada, Australia and some other former British colonies despite the official adoption (or partial adoption in the case of the UK) of the metric system. An alternative system of mass is generally used for precious materials, Troy weight.

The word avoirdupois is from Anglo-Norman French aveir de peis (later avoir de pois), literally "goods of weight" (Old French aveir, "property, goods", also "to have", comes from the Latin habere, "to have, to hold, to possess property"; de = "from"/"of", cf. Latin; peis = "weight", from Latin pensum). 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 it become identified with a particular system of units used to weigh such merchandise. The orthography of the day 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.)