The Collaborative International Dictionary of English v.0.48:

Codon \Co"don\ (k[=o]"d[o^]n), n. (Molecular biology)
   a sequence of three nucleotides in a genome or a DNA or
   messenger RNA molecule, which specifies the incorporation of
   one amino acid or is a stop signal, during the biosynthesis
   of proteins. Codons occur within the protein-coding segments
   of the DNA or RNA genome of living organisms. The amino acid
   sequence of proteins synthesized on ribosomes is thus
   determined by the sequence of the nucleotides in the genome.
   [PJC]

   Note: Outside of coding regions, there is no direct
         correspondence of the nucleotide sequence with protein
         sequence, and certain signalling functions of nucleic
         acids are not specified by three-nucleotide codons.
         Certain types of polypeptide synthesized in living
         organisms are not synthesized on ribosomes, and the
         sequences of these polypeptides do not have a
         corresponding nucleotide sequence in the genome.
         [PJC]

WordNet (r) 3.0 (2006):

codon
    n 1: a specific sequence of three adjacent nucleotides on a
         strand of DNA or RNA that specifies the genetic code
         information for synthesizing a particular amino acid