Free radical is a chemistry term that describes an atomic or molecular species with unpaired electrons on an otherwise open shell configuration.

In chemistry, a radical (more precisely, a free radical) is an atom, molecule, or ion that has unpaired valence electrons. With some exceptions, these unpaired electrons make free radicals highly chemically reactive towards other substances, or even towards themselves: their molecules will often spontaneously dimerize or polymerize if they come in contact with each other. Most radicals are reasonably stable only at very low concentrations in inert media or in a vacuum.