From 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]