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

Formulated with skin-nourishing ingredients like vitamin C, collagen, and sodium hyaluronate (a sister to hyaluronic acid), the serum reduces signs of aging and supports collagen production, evens your skin complexion, and defends against free radicals. —Alicia Geigel, Southern Living, 27 Nov. 2025

Reduces Stress and Inflammation Spinach also has antioxidants, flavonoids, and phytochemicals that protect your cells from oxidative stress and damage from free radicals. —Abby Norman, Verywell Health, 26 Nov. 2025

The star ingredients here are oil-absorbing clays (including kaolin and bentonite) and the brand’s signature grapeseed polyphenols, which provide free radical protection as well as combat sebum oxidation—the reason blackheads appear dark. —Emily Orofino, Vogue, 21 Nov. 2025

The echinacea plant is packed with potent antioxidants, and contains a wide net of compounds that are thought to help reduce free radicals and combat inflammation in the body, says Megan Ramaika, a bariatric dietitian at Medstar Washington Hospital Center. —Caroline C. Boyle, USA Today, 21 Nov. 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