Molar concentration, or molarity, or simply concentration, is a term in physical chemistry. It measures the concentration of a solution or mixture.

In chemistry, the molar concentration, \displaystyle{ c_i } is defined as the amount of a constituent \displaystyle{ n_i } (usually measured in moles – hence the name) divided by the volume of the mixture \displaystyle{ V }:

\displaystyle{ c_i = \frac {n_i}{V} }

The volume \displaystyle{ V } in the definition \displaystyle{ c_i = n_i/V } refers to the volume of the solution, not the volume of the solvent. One litre of a solution usually contains either slightly more or slightly less than 1 litre of solvent because when a substance dissolves in a solvent it causes volume of liquid to increase or decrease.