The ratio of the analytical concentration of a component in the stationary phase to its analytical concentration in the mobile phase. \[D_{\text{c}}=\frac{\text{amount of component}/\text{cm}^{3}\text{of stationary phase}}{\text{amount of component}/\text{cm}^{3}\text{of mobile phase}}\]
Source:
Orange Book, 2nd ed., p. 106 (https://media.iupac.org/publications/analytical_compendium/)