WordNet (r) 3.0 (2006):

free radical
    n 1: an atom or group of atoms with at least one unpaired
         electron; in the body it is usually an oxygen molecule that
         has lost an electron and will stabilize itself by stealing
         an electron from a nearby molecule; "in the body free
         radicals are high-energy particles that ricochet wildly and
         damage cells" [syn: free radical, radical]