The atom (or group of atoms) in a polyatomic radical on which an unpaired electron
is largely localized.
Attachment of a monovalent atom to a radical centre gives a molecule for which it is possible
to write a
Lewis formula in which the normal
stable valencies are assigned to all atoms.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1156