Normally an inert porous solid, which sorbs the liquid phase. The particle-size range of the support affects column efficiency and the pressure differential necessary to achieve a given flow rate. Modifications have been introduced for the achievement of special separations, in which the solid support is not inert but is an active solid. In capillary columns the inner wall of the column serves as the solid support and obviates the use of additional porous solids for this purpose.
Orange Book, 2nd ed., p. 98 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 830 (https://doi.org/10.1351/pac199365040819)