The mole (symbol mol) is the SI unit for the quantity amount of substance (symbol n). It is defined as the amount of substance that contains as many of the specified entities as there are ^{12}C atoms in 0.012 kg of ^{12}C. It is essential to specify the entities in question, such as atoms, molecules, ions, molecular fragments, electrons, protons, etc., or any defined collection of such entities. It is therefore possible to specify the amount of substance such as 1 mol of
, 1 mol of 2H^{+}, or 1 mol of H_{2}SO_{4}. It is also correct to specify, for example, 1 mol of 1/2 Cl_{2} or 1 mol of CH_{3}.

The actual number of entities present in amount of substance n is of course equal to Ln where L = 6.0221367 × 10^{23} mol^{−1} is Avogadro's constant (Avogadro Number). Similarly, the mass of material present in amount of substance n is Mn where M is the molar mass of the specified entities. Thus, the amount of substance can frequently be determined from mass measurement.