pound, abbr. lb, unit of either mass or force in the customary system of English units of measurement. Two different pounds of mass are defined, one in the avoirdupois system of units and one in the Troy system. The avoirdupois pound (lb avdp) is now defined in terms of the kilogram, the metric unit of mass; 1 lb avdp is equal to 0.45359237 kg. The Troy pound is used only for the measurement of precious metals and is defined as 5760/7000 of the avoirdupois pound. The apothecaries' pound is identical to the Troy pound. As a unit of force, or weight, the pound is the weight that a mass of 1 lb avdp has when the acceleration of gravity has its standard value (9.80665 meters per second per second). In ordinary usage, the term pound is often used without specifying whether force or mass is meant, but for scientific purposes it is important to make this distinction.
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