A defined linked @C01148@ of atoms or a single atom within a @M03986@. 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)
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1118 [Terms] [Paper]