The plane ideally marking the boundary between two phases.
The term should not be used synonymously with interphase.
PAC, 1986, 58, 437 (Interphases in systems of conducting phases (Recommendations 1985)) on page 439
See also:
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 583
Orange Book, p. 251