Definition of radical

• (chemistry) two or more atoms bound together as a single unit and forming part of a molecule
• an atom or group of atoms with at least one unpaired electron; in the body it is usually an oxygen molecule than 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"
• a person who has radical ideas or opinions