surface of a phase (free surface)

https://doi.org/10.1351/goldbook.S06182
The plane ideally marking the boundary between a phase and its environment.
Source:
PAC, 1983, 55, 1251. 'Interphases in systems of conducting phases (Provisional)' on page 1252 (https://doi.org/10.1351/pac198355081251)