A film thinner than about 1/4 wavelength of visible light. Black films are often equilibrium films, but equilibrium films may be considerably thicker under some conditions. In soap films, two types of equilibrium films are often observed: the one characterized by thickness of the order of 7 nm or more which varies significantly with minor changes in composition such as ionic strength is called common black film.
See also: Newton black film
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)