Any constitutionally or isotopically distinct atom, molecule, ion, @[email protected], radical, @[email protected], complex, @[email protected] etc., identifiable as a separately distinguishable entity. Molecular entity is used in this Compendium as a general term for singular entities, irrespective of their nature, while @[email protected] stands for sets or ensembles of molecular entities. Note that the name of a compound may refer to the respective molecular entity or to the chemical species, e.g. methane, may mean a single molecule of CH4 (molecular entity) or a molar amount, specified or not (chemical species), participating in a reaction. The degree of @[email protected] necessary to describe a molecular entity depends on the context. For example 'hydrogen molecule' is an adequate definition of a certain molecular entity for some purposes, whereas for others it is necessary to distinguish the @[email protected] and/or vibrational state and/or nuclear spin, etc. of the hydrogen molecule.