A fluid which percolates through or along the stationary bed, in a definite direction. It may be a liquid (liquid chromatography) or a gas (gas chromatography) or a supercritical fluid (supercritical-fluid chromatography). In gas chromatography the expression carrier gas may be used for the mobile phase. In elution chromatography the expression 'eluent' is also used for the mobile phase.
Sources:
Orange Book, 2nd ed., p. 97 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2202 (https://doi.org/10.1351/pac199062112167)
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 824 (https://doi.org/10.1351/pac199365040819)