The chromatographic bed used in thin-layer chromatography in which there is a gradual transition in some property.
Source:
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 830 (https://doi.org/10.1351/pac199365040819)
See also:
Orange Book, 2nd ed., p. 98 (https://media.iupac.org/publications/analytical_compendium/)