The reversible work of formation of unit area of new surface by cleavage. It has previously been called specific surface work, surface energy, surface tension or surface free energy. The symbol recommended is γ π in preference to σ because of the confusion with surface charge density. The superficial work γ π is a scalar quantity and consequently is isotropic.
PAC, 1983, 55, 1251. 'Interphases in systems of conducting phases (Provisional)' on page 1265 (https://doi.org/10.1351/pac198355081251)