Any conceptual region of space restricted in one dimension, within or at the surface of a condensed phase or a film. The usage of the term 'film' for an adsorption layer is confusing and is discouraged. The term double-layer applies to layers approximated by two 'distinct' sublayers.
Source:
PAC, 1994, 66, 1667. 'Thin films including layers: terminology in relation to their preparation and characterization (IUPAC Recommendations 1994)' on page 1671 (https://doi.org/10.1351/pac199466081667)