The fraction of the sample component in the mobile phase at equilibrium; it is related to the retention factor and other fundamental chromatography terms: \[R = \frac{1}{k\,+\,1}\]
Source:
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 843 (https://doi.org/10.1351/pac199365040819)