The mole (symbol mol) is a unit of measurement,  the base unit in the International System of Units (SI) for amount of substance, a quantity proportional to the number of elementary entities of a substance. One mole contains exactly  elementary entities (approximately 602 sextillion or 602 billion times a trillion), which can be atoms, molecules, ions, or other particles. The number of particles in a mole is the Avogadro number (symbol N) and the numerical value of the Avogadro constant (symbol N) expressed in mol. The value was chosen based on the historical definition of the mole as the amount of substance that corresponds to the number of atoms in 12 grams of C, which made the mass of a mole of a compound expressed in grams numerically equal to the average molecular mass of the compound expressed in daltons. With the 2019 redefinition of the SI base units, the numerical equivalence is now only approximate but may be assumed for all practical purposes.
The mole is widely used in chemistry as a convenient way to express amounts of reactants and amounts of products of chemical reactions. For example, the chemical equation  can be interpreted to mean that for each 2 mol molecular hydrogen (H) and 1 mol molecular oxygen (O) that react, 2 mol of water (HO) form. The concentration of a solution is commonly expressed by its molar concentration, defined as the amount of dissolved substance per unit volume of solution, for which the unit typically used is mole per litre (mol/L).