In a protein coding sequence of DNA, three subsequent nucleotides form codons (see figure 1), which, in turn, form amino acids that will eventually give rise to a protein. Some of these codons, however, are synonymous, meaning that one amino acid can be encoded by several different codons. When an amino acid is preferentially encoded by one of several potential synonymous codons, it is called codon bias.
Figure 1: Example of a codon. (GCA codes for alanine, which is also encoded by the synonymous codons GCT, GCT and GCG.)
(Source: Georgia Southern University, Department of Biology.)