Concentration of a substance in one phase divided by the concentration of the substance in the other phase when the heterogeneous system of two phases is in equilibrium.

Note 1: The ratio of concentrations (or, strictly speaking, activities) of the same molecular species in the two phases is constant at constant temperature.

Note 2: The octanol/water partition coefficient is often used as a measure of the bioconcentration factor for modeling purposes.

Note 3: This term is in common usage in toxicology but is not recommended by IUPAC for use in chemistry and should not be used as a synonym for partition constant, partition ratio or distribution ratio.