Osmolarity, formerly known as osmotic concentration, is a measure of solute concentration in a solution, defined as the number of osmoles of solute per liter of solution. The osmolarity of a solution is usually expressed as Osm/L (pronounced "osmolar"), in the same way that the molarity of a solution is expressed as "M" (pronounced "molar"). Osmolarity is distinct from molarity because it measures osmoles of solute particles rather than moles of solute.

Osmolality is another expression of concentration reflecting the osmoticity of solutions. It is the measure of the concentration of particles in the serum per kilogram of water. Osmolality and osmolarity are related but differ in the way they measure concentration. Osmolarity measures the concentration per liter of water, while osmolality measures the concentration per kilogram of water.

The unit of measurement for osmolarity is the osmol (Osm), while the unit of measurement for osmolality is the osmol/kg. Osmolarity can be used to predict whether water will move from one side of a semipermeable membrane to the other, which is the basis of osmosis.

In summary, osmolarity is a measure of solute concentration in a solution, expressed as the number of osmoles of solute per liter of solution. It is distinct from molarity because it measures osmoles of solute particles rather than moles of solute. Osmolality is another expression of concentration reflecting the osmoticity of solutions, which measures the concentration of particles in the serum per kilogram of water.