The distribution constant (or partition ratio) (K) is the equilibrium constant for the distribution of an analyte in two immiscible solvents.
In chromatography, for a particular solvent, it is equal to the ratio of its molar concentration in the stationary phase to its molar concentration in the mobile phase, also approximating the ratio of the solubility of the solvent in each phase. 
The term is often confused with partition coefficient or distribution coefficient.