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

The key ingredients here are biotin, keratin, marine collagen, coriander, and patchouli. —Tamim Alnuweiri, InStyle, 16 Jan. 2026

With hydrolyzed silk and hydrolyzed keratin to strengthen and repair, the spray provides all-day hold while keeping hair touchable and soft. —Conçetta Ciarlo, Vogue, 15 Jan. 2026

The silky conditioner pairs keratin with jojoba seed oil to soften hair and boost shine, leaving strands glossy enough to pass for a fresh salon blowout. —Lily Wohlner, Allure, 11 Jan. 2026

The secret weapon in question is bioactive keratin ferment, which manages to match proteins native to human hair while delivering vital nutrients straight to each strand’s cortex. —Stacia Datskovska, Footwear News, 27 Dec. 2025

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