Source : Webster's Revised Unabridged Dictionary (1913)

Chitin \Chi"tin\, n. [See {Chiton}.] (Chem.)
   A white amorphous horny substance forming the harder part of
   the outer integument of insects, crustacea, and various other
   invertebrates; entomolin.

Source : WordNet®

chitin
     n : a tough semitransparent horny substance; the principal
         component of the exoskeletons of arthropods and the cell
         walls of certain fungi