Codon adaptation index is a measurement of the relative adaptiveness of the codon usage of a gene towards the codon usage of highly expressed genes. The relative adaptiveness (w) of each codon is the ratio of the usage of each codon, to that of the most abundant codon for the same amino acid. The CAI index is defined as the geometric mean of these relative adaptiveness values. Non-synonymous codons and termination codons (dependent on genetic code) are excluded. CAI values range from 0 to 1, with higher values indicating a higher proportion of the most abundant codons. [Sharp, P. M., and W. H. Li , (1987). The codon adaptation index a measure of directional synonymous codon usage bias, and its potential applications. Nucleic Acids Research 15: 1281-1295. Abstract, also see: Jansen R., Bussemaker H.J., and Gerstein M. (2003) Revisiting the codon adaptation index from a whole-genome perspective: analyzing the relationship between gene expression and codon occurrence in yeast using a variety of models. Nucleic Acids Res. 31(8):2242-51. Abstract ]

This definition is in context to Yeast Genome. See more contextual defintions for Codon Adaptation Index (CAI).