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: \[\phi =\frac{\mu _{\rm{A}}^{*}- \mu _{\rm{A}}}{R\ T\ M_{\rm{A}}\ \sum _{\begin{array}{c} i \end{array}}m_{i}}\] and on an amount fraction basis by: \[\phi =\frac{\mu _{\rm{A}}^{*}- \mu _{\rm{A}}}{R\ T\ \ln x_{\rm{A}}}\] where \(\mu _{\rm{A}}^{*}\) and \(\mu _{\rm{A}}\) are the chemical potentials of the solvent as a pure substance and in solution, respectively, \(M_{\rm{A}}\) is its molar mass, \(x_{\rm{A}}\) its amount fraction, \(R\) the gas constant and \(T\) the temperaure. The latter osmotic coefficient is sometimes called the rational osmotic coefficient.