The n-octanol-water partition coefficient, K is a partition coefficient for the two-phase system consisting of n-octanol and water. K is also frequently referred to by the symbol P, especially in the English literature. It is also called n-octanol-water partition ratio. 
K serves as a measure of the relationship between lipophilicity (fat solubility) and hydrophilicity (water solubility) of a substance. The value is greater than one if a substance is more soluble in fat-like solvents such as n-octanol, and less than one if it is more soluble in water.
If a substance is present as several chemical species in the octanol-water system due to association or dissociation, each species is assigned its own K value. A related value, D, does not distinguish between different species, only indicating the concentration ratio of the substance between the two phases.