The ratio of the volume of the mobile phase to that of the stationary phase in a column: \[\beta = \frac{V_{0}}{V_{\text{s}}}\] In the case of open-tubular columns the geometric internal volume of the tube (V c) is to be substituted for V 0.
Sources:
Orange Book, 2nd ed., p. 100 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 833 (https://doi.org/10.1351/pac199365040819)