The force Π d per unit area which can be obtained as the derivative of -gt with respect to the distance, where gt is the total Gibbs energy of interaction per unit area of each of the two flat and parallel plates.
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 616 (https://doi.org/10.1351/pac197231040577)