The work required to form unit area of new surface by stretching under equilibrium conditions. It is numerically equal to the force acting in the j
th direction per unit length of exposed edge, the edge being normal to the i
th direction, that must be applied to a terminating surface to keep it in equilibrium, the i
th and j
th directions lying in the plane of the surface.
PAC, 1986, 58, 437. 'Interphases in systems of conducting phases (Recommendations 1985)' on page 451 (https://doi.org/10.1351/pac198658030437)