A codon is a sequence of three nucleotides in DNA or RNA that represents a specific amino acid or a stop signal. It acts as a "code" that tells the cell which amino acid to add to a growing protein chain during the process of protein synthesis.
This approach will allow you, for example, to insert gene tags, loxP sites for conditional models and stop codons in specific functional regions. (jax.org)
This study showed that these genes may be pseudogenes with early stop codons in the baleen whales. (sciencedaily.com)
Also, the order of codons in the gene specifies the order of amino acids in the protein. (science.howstuffworks.com)