A single, closely packed layer of atoms or molecules. The term floating monolayer is used for certain spread monolayers or films which are in the condensed — often solid — state. The term Langmuir monolayer has recently been coined for spread monolayers. This usage is not recommended.
Source:
PAC, 1994, 66, 1667. 'Thin films including layers: terminology in relation to their preparation and characterization (IUPAC Recommendations 1994)' on page 1672 (https://doi.org/10.1351/pac199466081667)
See also:
Orange Book, 2nd ed., p. 251 (https://media.iupac.org/publications/analytical_compendium/)