unit, unit of measurement - any division of quantity accepted as a standard of measurement or exchange; "the dollar is the United States unit of currency"; "a unit of wheat is a bushel"; "change per unit volume"

atomic mass unit - unit of mass for expressing masses of atoms or molecules

mass number, nucleon number - the sum of the number of neutrons and protons in an atomic nucleus

avoirdupois unit - any of the units of the avoirdupois system of weights

metric weight unit, weight unit - a decimal unit of weight based on the gram