Osmolarity, more commonly known as osmotic concentration, is the measure of the concentration of a solute. This unit of this measurement is written as the number of osmoles of the solute, the dissolved substance of a solution, per liters of solution.
Osmoles are a non-standard unit of measurement of the number of moles of solute that act to maintain a stable osmotic pressure in the solution. Stable osmotic pressure means that the solvent of a solution is not taken in or expelled from cells or other semi-permeable membranes. For example, a solution of sodium chloride in water results in a certain number of moles of salt, but double the number of osmoles of salt. This is due to the fact that salt splits into two ions in the presence of water, creating double the number of particles.