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]