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 (electron dot or Lewis structure) 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 (https://doi.org/10.1351/pac199466051077)