A defined linked
collection of atoms or a single atom within a
molecular entity. This use of the term in physical organic and general chemistry is less restrictive than the definition adopted for the purpose of nomenclature of organic compounds.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1118
See also:
Blue Book, p. 82Blue Book, p. 85