Medical Definition of OSMOLARITY. : the concentration of an osmotic solution especially when measured in osmols or milliosmols per liter of solution.— os·mo·lar \aez-äz-ˈmō, lər-\ aes. äs adjectiveedical Definition of OSMOLARITY. : the concentration of an osmotic solution especially when measured in osmols or milliosmols per liter of solution.