An equivalent (symbol: officially equiv; unofficially but often Eq) is the amount of a substance that reacts with (or is equivalent to) an arbitrary amount (typically one mole) of another substance in a given chemical reaction. It is an archaic unit of measurement that was used in chemistry and the biological sciences (see Equivalent weight#In history). The mass of an equivalent is called its equivalent weight.

In a more formal definition, the equivalent is the amount of a substance needed to do one of the following:

By this definition, the number of equivalents of a given ion in a solution is equal to the number of moles of that ion multiplied by its valence. If 1 mol of NaCl and 1 mol of CaCl2 dissolve in a solution, there is 1 equiv Na, 2 equiv Ca, and 3 equiv Cl in that solution. (The valency of calcium is 2, so for that ion 1 mole is 2 equivalents.)

An earlier definition, used especially for chemical elements, holds that an equivalent is the amount of a substance that will react with 1 g (0.035 oz) of hydrogen, 8 g (0.28 oz) of oxygen, or 35.5 g (1.25 oz) of chlorine—or that will displace any of the three.