free radical

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An atom or group of atoms that has at least one unpaired electron and is therefore unstable and highly reactive. In animal tissues, free radicals can damage cells and are believed to accelerate the progression of cancer, cardiovascular disease, and age-related diseases.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry (physics) any molecule, ion or atom that has one or more unpaired electrons; they are generally highly reactive and often only occur as transient species

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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