free radical n an esp. reactive atom or group of atoms that has one or more unpaired electrons esp one that is produced in the body by natural biological processes or introduced from outside (as in tobacco smoke, toxins, or pollutants) and that can damage cells, proteins, and DNA by altering their chemical structure
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a radical that carries an unpaired electron; such radicals are extremely reactive, with a very short half-life (10−5 second or less in an aqueous solution).Medical dictionary. 2011.