The mechanical properties of an
interfacial layer between two fluid phases may be expressed in terms of those of a geometrical surface
of uniform tension called the surface of tension, whose location is dependent on the
distribution of the stress tensor within the
interfacial layer.
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 596