osmotic coefficient, ϕ

Quantity characterizing the deviation of the solvent from ideal behaviour referenced to Raoult's law. The osmotic coefficient on a molality basis is defined by:
ϕ = μ A * − μ A R T M A ∑ i m i
and on an amount fraction basis by:
ϕ = μ A * − μ A R T ln x A
where μ A * and μ A are the chemical potentials of the solvent as a pure substance and in solution, respectively, M A is its molar mass, x A its amount fraction, R the gas constant and T the temperaure. The latter osmotic coefficient is sometimes called the rational osmotic coefficient.
PAC, 1994, 66, 533 (Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994)) on page 546