A codon is a sequence of three nucleotides (a trinucleotide) that forms a unit of genomic information encoding a particular amino acid or signaling the termination of protein synthesis (stop signals) . Codons are found in DNA and RNA, and they correspond to specific amino acids or stop signals during protein synthesis. There are 64 different codons, with 61 specifying amino acids and 3 used as stop signals.

Each codon instructs the cell to start the creation of a protein chain, to add a specific amino acid to the growing protein chain, or to stop creation of the protein chain. For example, a messenger RNA codon, GCA, signals the addition of the amino acid alanine to the protein chain, while the messenger RNA stop codon, UAG, signals the end of that proteins production.

Codons are read during translation, beginning with a start codon and continuing until a stop codon is reached. mRNA codons are read from 5 to 3, and they specify the order of amino acids in a protein from N-terminus (methionine) to C-terminus. The full set of relationships between codons and amino acids (or stop signals) is called the genetic code.