A procedure in which the composition of the mobile phase is changed continuously or stepwise during the elution process.
Sources:
Orange Book, 2nd ed., p. 93 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 826 (https://doi.org/10.1351/pac199365040819)