free radical

noun

: an especially reactive atom or group of atoms that has one or more unpaired electrons

especially : one that is produced in the body by natural biological processes or introduced from an outside source (such as tobacco smoke, toxins, or pollutants) and that can damage cells, proteins, and DNA by altering their chemical structure

Examples of free radical in a Sentence

Recent Examples on the Web

Whitening strips blend both the natural process of oil pulling with hydrogen peroxide to safely and effectively release oxygen and hydroxyl free radicals for a brighter, whiter smile in as soon as 14 days. —ABC News, 1 May 2026

Antioxidants protect the body from free radicals, unstable molecules that cause cell damage. —Carrie Madormo, Verywell Health, 1 May 2026

Scientists use a technique called electron paramagnetic resonance (EPR) spectroscopy to pick up the concentration and identities of free radicals, but today’s equipment relies on huge, heavy magnets. —IEEE Spectrum, 30 Apr. 2026

Plus, as a powerful antioxidant, vitamin C clears away cell-damaging compounds called free radicals. —Mark Gurarie, Health, 27 Apr. 2026

Word History

First Known Use

1870, in the meaning defined above

Rhymes for free radical

• radicle

Medical Definition

free radical

noun

: an especially reactive atom or group of atoms that has one or more unpaired electrons

especially : one that is produced in the body by natural biological processes or introduced from outside (as in tobacco smoke, toxins, or pollutants) and that can damage cells, proteins, and DNA by altering their chemical structure