The Collaborative International Dictionary of English v.0.48:

Leucine \Leu"cine\ (l[=oo]"s[=e]n), Leucin
\Leu"cin\(l[=u]"s[i^]n), n. [Gr. leyko`s white.] (Physiol.
   Chem.)
   a naturally occurring alpha-amino acid
   ((CH3)2CH.CH2.CH(NH2)-COOH), one of the building units of
   almost all proteins of living organisms, both animal and
   vegetable. It is one of the essential amino acids (not
   synthesized by the human body, a required component for
   proper nutrition), and is hydrophobic in character when bound
   in proteins. In isolated form it is a white, crystalline,
   zwitterionic substance formed, e. g. by the decomposition of
   proteins by pancreatic digestion, by the action of boiling
   dilute mineral acid, or by putrefaction. Chemically it is to
   be considered as amido-caproic acid. It occurs as two optical
   isomers, the L- and D-forms. The L-form, L-leucine, is the
   natural form, present in most proteins.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

leucine
    n 1: a white crystalline amino acid occurring in proteins that
         is essential for nutrition; obtained by the hydrolysis of
         most dietary proteins