critical thickness
of a film
A film often thins gradually to a thickness at which it either ruptures or converts abruptly to an equilibrium film. This thickness is sometimes well enough defined statistically to be considered a critical thickness, Math - text or Math - text.
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