A sorption isotherm (also adsorption isotherm) describes the equilibrium of the sorption of a material at a surface (more general at a surface boundary) at constant temperature. It represents the amount of material bound at the surface (the sorbate) as a function of the material present in the gas phase and/or in the solution. Sorption isotherms are often used as empirical models, which do not make statements about the underlying mechanisms and measured variables. They are obtained from measured data by means of regression analysis. The most frequently used isotherms are the linear isotherm, Freundlich isotherm, the Langmuir isotherm, and the BET model.