A geometrical surface chosen parallel to the interface and used to define the volumes of the bulk phases in the calculation of the extent of adsorption, and of other surface excess properties. Also referred to as Gibbs dividing surface.
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 588 (https://doi.org/10.1351/pac197231040577)