Mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as the amount of any substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon-12 (12C), the isotope of carbon with relative atomic mass 12. 

This corresponds to the Avogadro constant, which has a value of 6.02214129(27)×1023 elementary entities of the substance. It is one of the base units in the International System of Units, and has the unit symbol mol and corresponds with the dimension symbol N. 

 In honour of the unit, some chemists celebrate October 23 (a reference to the 1023 part of Avogadro's number) as "Mole Day".

The mole is widely used in chemistry instead of units of mass or volume as a convenient way to express amounts of reactants or of products of chemical reactions. 

For example, the chemical equation 2 H2 + O2 → 2 H2O implies that 2 mol of dihydrogen (H2) and 1 mol of dioxygen (O2) react to form 2 mol of water (H2O). 

The mole may also be used to express the number of atoms, ions, or other elementary entities in a given sample of any substance. The concentration of a solution is commonly expressed by its molarity, defined as the number of moles of the dissolved substance per litre of solution.

The number of molecules in a mole (known as Avogadro's constant) is defined such that the mass of one mole of a substance, expressed in grams, is exactly equal to the substance's mean molecular mass. 

For example, the mean molecular mass of natural water is about 18.015, so one mole of water is about 18.015 grams. 

Making use of this equation considerably simplifies many chemical and physical computations.

Former terms for amount of substance

The term gram-molecule was formerly used for essentially the same concept. The term gram-atom (abbreviated gat.) has been used for a related but distinct concept, namely a quantity of a substance that contains Avogadro's number of atoms, whether isolated or combined in molecules. Thus, for example, 1 mole of MgB2 is 1 gram-molecule of MgB2 but 3 gram-atoms of MgB2.