Defined by \[G^{\sigma }=H^{\sigma }- T\ S^{\sigma }=A^{\sigma }- \gamma \ A_{\text{s}}\] where Hσ is the surface excess enthalpy, T is the thermodynamic temperature, S σ is the surface excess entropy, Aσ is the surface excess Helmholtz energy, γ is the surface tension, and As is the area relative to a Gibbs 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 599 (https://doi.org/10.1351/pac197231040577)