A condensed phase at the surface of which adsorption may occur.
Sources:
Orange Book, 2nd ed., p. 85 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2171 (https://doi.org/10.1351/pac199062112167)