The material that is present in one or other (or both) of the bulk phases and capable of being adsorbed.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 584 (https://doi.org/10.1351/pac197231040577)