keratin

[ker-uh-tin]
noun
a scleroprotein or albuminoid substance, found in the dead outer skin layer, and in horn, hair, feathers, hoofs, nails, claws, bills, etc.

Origin:
1840–50; kerat- + -in2

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
keratin or ceratin (ˈkɛrətɪn)
 
n
a fibrous protein that occurs in the outer layer of the skin and in hair, nails, feathers, hooves, etc
 
ceratin or ceratin
 
n

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Etymonline
Word Origin & History

keratin
"basic substance of horns, nails, feathers, etc.," 1847, from Gk. keras (gen. keratos) "horn" + chemical suffix -in.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

keratin ker·a·tin (kěr'ə-tĭn)
n.
Any of a group of scleroproteins or albuminoids that contain large amounts of sulfur and are the chief structural constituents of hair, nails, and other horny tissues.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
keratin   (kěr'ə-tĭn)  Pronunciation Key 
Any of a class of tough, fibrous proteins that are the main structural component of hair, nails, horns, feathers, and hooves. Keratins are rich in sulfur-containing amino acids, especially cysteine. Individual keratin molecules are entwined helically around each other in long filaments, which are cross-linked by bonds between sulfur atoms on different chains. The twining and cross-linking produce strength and toughness.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Feathers are composed of keratin, a fibrous protein that has the same sort of
  strength as synthetic fibres.
It is known by the name of the enamel droplet, and resembles keratin in its
  resistance to the action of mineral acids.
They are composed not of keratin, the stuff of fingernails, but of bone.
The dark hair pigments are the first to decompose leaving the lighter pigments
  and lighter in color keratin.
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