The Collaborative International Dictionary of English v.0.48:

Serine \Ser"ine\ (s[e^]r"[=e]n; s[e^]r"[i^]n; also, less
   correctly, s[=e]r"[=e]n), n. [L. sericus silken.] (Chem.)
   one of the natural L-amino acids, obtainable as a white
   crystalline nitrogenous substance by the action of dilute
   sulphuric acid on silk gelatin. It is found in many proteins,
   and, having a free primary hydroxyl group on the side chain,
   is involved in the catalytic action at the active site of
   some enzymes, such as proteases. The IUPAC abbreviation for
   serine in protein sequences is Ser. Chemically it is
   2-amino-3-hydroxy-propanoic acid (C3H7NO3),
   HO.CH2.CH(NH2).COOH.
   [1913 Webster +PJC] Serio-comic

WordNet (r) 3.0 (2006):

serine
    n 1: a sweetish crystalline amino acid involved in the synthesis
         by the body of cysteine