keratin

noun

ker·​a·​tin ˈker-ə-tən 

: any of various sulfur-containing, fibrous, acidic or basic proteins chiefly of epithelial cells and tissues that are relatively insoluble and resistant to degradation, form filaments which assemble into bundles to provide structural support, and are the primary component of hair, nails, horns, claws, hooves, quills, scales, feathers and the epidermal layer of skin

Note: In addition to its structural properties, keratin also plays a role in cell growth and proliferation, cellular transport of substances, and intercellular communication.

Rhino horn is not actually horn but densely compacted fibers of keratin, a protein found in hair and fingernails.

—Maryanne Vollers

Wool fibers are composed of keratin proteins bundled into microfibrils.

—A. Goho

compare keratinocyte

keratinous

kə-ˈra-tə-nəs 

adjective

Examples of keratin in a Sentence

Recent Examples on the Web

Virtue’s whole line runs on one ingredient, Alpha Keratin 60ku, a protein the brand says is identical to the keratin your hair is made of. — Valeriya Chupinina, Vogue, 4 Nov. 2026

The chemicals that make up the substances became embedded in the keratin of the hair and nails, which then became preserved over the centuries, meaning they could be identified through toxicological analysis. — Adam England, PEOPLE, 3 Nov. 2026

This semi-permanent glaze softens brassiness on all hair colors and simultaneously conditions with keratin, argan oil, and ginseng root extract. — Ariel Wodarcyk, InStyle, 3 Nov. 2026

It’s topped off with a two-in-one purple conditioner that pairs anti-brass benefits with a hydrating keratin complex for stronger strands. — Jennifer Hussein, Allure, 1 Nov. 2026

Word History

Etymology

borrowed from German Keratin, from Greek kerat-, stem of kéras "horn" + German -in -in entry 1 — more at kerato-

Note: The term was introduced by the German physician and chemist Johann Franz Simon (1807-43) in Handbuch der angewandten medizinischen Chemie, 1. Theil, Medizinisch-analytische Chemie (Berlin, 1840), p. 49.

First Known Use

circa 1849, in the meaning defined above

Rhymes for keratin

• ferritin
• cytokeratin
• samaritan

Kids Definition

keratin

noun

ker·​a·​tin ˈker-ət-ᵊn 

: a sulfur-containing protein that makes up hair and horny tissues (as nails)

Medical Definition

keratin

noun

ker·​a·​tin ˈker-ət-ən 

: any of various sulfur-containing fibrous proteins that form the chemical basis of horny epidermal tissues (as hair and nails) and are typically not digested by enzymes of the gastrointestinal tract see pseudokeratin