A column, usually having a small diameter, in which either the inner tube wall, or a liquid or active solid held stationary on the tube wall acts as the stationary phase and there is an open, unrestricted path for the mobile phase.
Sources:
Orange Book, 2nd ed., p. 97 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 831 (https://doi.org/10.1351/pac199365040819)