Wordnet 3.0
NOUN (1)
1. 
 a crystalline amino acid found in proteins (especially keratin); 
 discovered in bladder stones; 
The Collaborative International Dictionary of English v.0.48:
Cystine \Cyst"ine\ (s?s"t?n; 104), n. [See Cyst.] (Physiol.
   Chem.)
   A white crystalline substance, C3H7NSO2, containing
   sulphur, occuring as a constituent of certain rare urinary
   calculi, and occasionally found as a sediment in urine.
   [1913 Webster]
WordNet (r) 3.0 (2006):
cystine
    n 1: a crystalline amino acid found in proteins (especially
         keratin); discovered in bladder stones