The Collaborative International Dictionary of English v.0.48:

Elastin \E*las"tin\, n. [Elastic + -in.] (Physiol. Chem.)
   A nitrogenous substance, somewhat resembling albumin, which
   forms the chemical basis of elastic tissue. It is very
   insoluble in most fluids, but is gradually dissolved when
   digested with either pepsin or trypsin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

elastin
    n 1: a fibrous scleroprotein found in elastic tissues such as
         the walls of arteries