The unpaired electron density at a position of interest, usually at carbon, in a radical. It is often measured experimentally by electron paramagnetic resonance [EPR
(electron spin resonance)] spectroscopy through hyperfine coupling constants of the atom or an attached hydrogen.
See also: radical centre
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1166 (https://doi.org/10.1351/pac199466051077)