1) Chemicals whose molecular or ionic structure includes an unpaired ("free") electron, usually conferring high reactivity; in biological systems, most free radicals contain oxygen.

2) Highly reactive molecules with an unsatisfied electron valence pair. Free radicals are produced in both normal and pathological processes. They are proven or suspected agents of tissue damage in a wide variety of circumstances including radiation, damage from environment chemicals, and aging. Natural and pharmacological prevention of free radical damage is being actively investigated.