The Collaborative International Dictionary of English v.0.48:

keratin \ker"a*tin\, n. [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]

WordNet (r) 3.0 (2006):

keratin
    n 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: keratin, ceratin]