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

Oxidative stress occurs when unstable molecules called free radicals damage cells and tissues, including those in your heart and blood vessels. — Alexandria Nyembwe, Health, 16 July 2026

Harmful molecules called free radicals show up, certain enzymes start breaking down collagen and the immune system is activated. — Leslie Baumann, Miami Herald, 9 July 2026

The hybrid formula is powered by mineral filter zinc oxide and two chemical sunscreen ingredients, while vitamin E provides additional protection against free radicals, which contribute to signs of aging, hyperpigmentation, and skin cancer. — Sara Coughlin, Allure, 9 July 2026

These compounds help fight free radicals, which are unstable molecules that can harm DNA and lead to cell mutations. — Kathi Valeii Published, Verywell Health, 8 July 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