Source : Webster's Revised Unabridged Dictionary (1913)

Keratin \Ker"a*tin\, n. [Gr. ke`ras, -atos, horn.] (Physiol.
   Chem.)
   A nitrogenous substance, or mixture of substances, containing
   sulphur in a loose state of combination, and forming the
   chemical basis of epidermal tissues, such as horn, hair,
   feathers, 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 leucin and tyrosin, as does albumin. Called also
   {epidermose}.

Source : WordNet®

keratin
     n : 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}]