The interparticle volume of a packed column per unit column volume: \[\varepsilon = \frac{V_{0}}{V_{\text{c}}}\] It is also called the interstitial fraction of the column.
Source:
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 832 (https://doi.org/10.1351/pac199365040819)