The activity coefficient is a factor used in thermodynamics and physical chemistry to quantify the deviation of a solution's behavior from that of an ideal solution. It is defined as the ratio of the activity of a species to its concentration (or mole fraction in the case of ideal solutions).
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An activity coefficient expresses departure from a chosen ideal-mixture activity convention. In a concentration-based convention,The thermodynamic activity and coefficient are dimensionless; is the standard concentration. The reference convention fixes which ideal or limiting state has . A mole-fraction convention uses a different ideal activity and must not be mixed silently with the concentration convention.[ref]
IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry. Entries A00115 and A00116, activity and activity coefficient: chemical-potential definition and mixture reference
Substituting activities into an equilibrium constant separates concentration ratios from interaction corrections. For , the correction contains . Changing composition can therefore change an observed concentration quotient even when the thermodynamic constant at fixed temperature and reference state is unchanged. An activity coefficient can be greater or less than one; it is not a fraction of molecules assigned to an active population.