Osmolarity is defined as the number of particles per litre of solution, or more accurately, the number of moles in solution that contribute to the osmotic pressure exerted on a membrane during osmosis. It is therefore a quantity involved in the definition of osmotic pressure.
It is measured in osmoles per litre. An osmole is a mole of particles in solution. For example, a solution of 1 mol/L of NaCl corresponds to an osmolarity of 2 osmol/L. It must be remembered that not all osmoles are necessarily osmotically active particles.
Jacobus Henricus Van't Hoff was a Dutch chemist. He was awarded the Nobel prize for chemistry and explained osmotic pressure. © Wikipedia_public domain