Pertains to a film element of a soap film changing in area at constant mass and is the differential change of its surface tension with relative change in area A, \[E=A\ (\frac{\partial \sigma }{\partial A})_{T,p,n_{i}}\] where T is the thermodynamic temperature, p is the pressure, and n i is the amount of substance of the species i.
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 615 (https://doi.org/10.1351/pac197231040577)