The Collaborative International Dictionary of English v.0.48:

Tyrosin \Tyr"o*sin\, n. [Gr. ? cheese.] (Physiol. Chem.)
   A white crystalline nitrogenous substance present in small
   amount in the pancreas and spleen, and formed in large
   quantity from the decomposition of proteid matter by various
   means, -- as by pancreatic digestion, by putrefaction as of
   cheese, by the action of boiling acids, etc. Chemically, it
   consists of oxyphenol and amidopropionic acid, and by
   decomposition yields oxybenzoic acid, or some other benzol
   derivative. [Written also tyrosine.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

tyrosine
    n 1: an amino acid found in most proteins; a precursor of
         several hormones