functional group
Organic compounds are thought of as consisting of a relatively @U06567@ @B00574@, for example a chain of \(\text{sp}^{3}\) hybridized carbon atoms, and one or several functional groups. The functional group is an atom, or a group of atoms that has similar chemical properties whenever it occurs in different compounds. It defines the characteristic physical and chemical properties of families of organic compounds.
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1116 [Terms] [Paper]