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

That stress, which arises from an excess of free radicals and a dearth of anti-oxidants, causes cells to stop working and thus eventually die. —New Atlas, 4 Jan. 2026

Its triterpenes support collagen synthesis and neutralize free radicals—two actions that help soften the look of fine lines and maintain a smoother, more resilient complexion. —Alice Abbiadati, Vogue, 3 Jan. 2026

Sunflower Seeds Sunflower seeds are especially rich in vitamin E, a fat-soluble antioxidant that protects the body from cell damage caused by free radicals. —Cathy Cassata, Health, 29 Dec. 2025

In addition to these benefits, vitamin A—when paired with vitamin C—acts as a powerful antioxidant that protects from damage caused by free radicals. —Lana Barhum, Verywell Health, 28 Dec. 2025

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