The Collaborative International Dictionary of English v.0.48:

Collagen \Col"la*gen\, n. [Gr. ko`lla glue + -gen.] (Physiol.
   Chem.)
   The chemical basis of ordinary connective tissue, as of
   tendons or sinews and of bone. On being boiled in water it
   becomes gelatin or glue.
   [1913 Webster]

WordNet (r) 3.0 (2006):

collagen
    n 1: a fibrous scleroprotein in bone and cartilage and tendon
         and other connective tissue; yields gelatin on boiling