A liquid film of thickness, or thicknesses, at which it is stable or metastable with respect to small thickness changes. Unless the area of the film is small, its composition may not be the same over its area and the (metastable) equilibrium thickness may be characteristic of the local condition only.
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 614 (https://doi.org/10.1351/pac197231040577)