Chem.
the average weight of a molecule of an element or compound measured in units once based on the weight of one hydrogen atom taken as the standard or on 1 / 16 the weight of an oxygen atom, but after 1961 based on 1 / 12 the weight of the carbon-12 atom; the sum of the atomic weights of all the atoms in a molecule. Abbr.: mol. wt. Also called formula weight .
[ 1875-80 ]