osmotic coefficient, ϕ

https://dx.doi.org/10.1351/goldbook.O04342
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.
Source:
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