Any constitutionally or isotopically distinct atom, molecule, ion, @I03231@, radical, @R05073@, complex, @C01262@ 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 @CT01038@ 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 @P04799@ 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 @ET07026@ and/or vibrational state and/or nuclear spin, etc. of the hydrogen molecule.