From The Collaborative International Dictionary of English v.0.44 [gcide]:
keratin \ker"a*tin\, noun [Gr. ke'ras, -atos, horn.] (Physiol. Chem.) A sulfur-containing fibrous protein constituting the main structural protein of hard epidermal tissues, such as horn, hair, feathers, nails, claws, hoofs, and the like. It is an insoluble substance, and, unlike elastin, is not dissolved even by gastric or pancreatic juice. By decomposition with sulphuric acid it yields leucine and tyrosine plus various other acid-stable amino acids. The amino acid composition varies, but it usually has a high percentage of cystine, which stabilizes and insolubilizes the protein by forming intrachain linkages. A softer form of keratin is present in the epidermis and whalebone. Called also {epidermose}. [1913 Webster +PJC]
From WordNet (r) 2.0 [wn]:
noun
1: a fibrous scleroprotein that occurs in the outer layer of the skin and in horny tissues such as hair feathers nails and hooves [syn: {ceratin}]
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