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

Your body has its own antioxidant defense systems of enzymes and compounds to help maintain a balance between free radicals and antioxidants, but dietary antioxidants are also found in foods like fruits, vegetables, and spices, and supplements. —Jillian Kubala, Health, 6 Nov. 2025

Melatonin is also an antioxidant, and antioxidants help protect against damage to DNA by oxidative stress, an imbalance between free radicals and antioxidants in the body. —Kristen Rogers, CNN Money, 4 Nov. 2025

Including various antioxidant foods, such as Brussels sprouts, in your regular meal rotation can counteract the damaging effects of free radicals. —Lauren Panoff, Verywell Health, 3 Nov. 2025

Here, a selection of hard-working blue light sunscreens to add to your free radical-blocking regimen. —Emily Orofino, Vogue, 30 Oct. 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