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.
See also: characteristic group (in organic nomenclature), principal group (in inorganic nomenclature), substituent
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1118 (https://doi.org/10.1351/pac199466051077)
Blue Book, p. 82 (http://www.acdlabs.com/iupac/nomenclature/)