The equilibrium relation between the concentration of a sample component in the stationary phase cS, and in the mobile phase cM, expressed as a function cS = f.cM. In some versions of chromatography, a distribution isotherm can be seen as a partition isotherm, an adsorption isotherm, or a combination of these, depending on the mechanism of separation.
The relation can be influenced also by concentrations of other sample components. cS and cM are usually expressed per unit volume of the phase; cS may also be expressed per mass of the dry solid phase or per unit surface area.
PAC, 1996, 68, 1591. 'Nomenclature for non-linear chromatography (IUPAC Recommendations 1996)' on page 1592 (https://doi.org/10.1351/pac199668081591)