A codon is the basic unit of the genetic code. It consists of a nucleotide triplet in DNA or messenger RNA (mRNA) which codes for a specific amino acid. As there are 64 possible codons but only 20 amino acids in terrestrial organisms, some amino acids have more than one codon (known as degenerative codons). There are also 'stop' codons which serve to terminate a translation.

An anticodon is a sequence of three nucleotides at the end of a transfer RNA molecule that is complementary to, and base pairs with, an amino acid specifying codon in messenger RNA during protein translation.