It is a measure of the relative proportions of two or more quantities in a mixture compound. Concentrations may be expressed in a number of ways. The simplest is in terms of a component's percentage by weight or volume.

Mixtures of solids or liquids are frequently specified by weight-percentage concentrations, whereas mixtures of gases are usually specified by volume percentages.

Very low concentrations, such as those of various substances in the atmosphere, are expressed in parts per million. The molarity of a solution is the number of moles of solute per litre of solution.

The molality of a solution is the number of moles of solute per 1,000 grams of solvent. The mole fraction of a solution is the ratio of moles of solute to the total number of moles in the solution.