(1) the weight in grams of an element that combines with or replaces 1 g of hydrogen; (2) the atomic or molecular weight in grams of an atom or group of atoms involved in a chemical reaction divided by the number of electrons donated, taken up, or shared by the atom or group of atoms in the course of that reaction; (3) the weight of a substance contained in 1 L of 1 n solution; a variant of (1). SYN: combining weight, equivalent weight, equivalent (5).