A stop codon is an RNA codon that, when encountered by a ribosome during RNA translation, immediately terminates translation because there does not exist a molecule of tRNA whose anticodon will base pair with the stop codon. A stop codon is also known as a "nonsense codon" because translation terminates for the simple reason that the ribosome does not know how to proceed.

The three possible stop codons are $\textrm{UAG}$, $\textrm{UGA}$, and $\textrm{UAA}$, which are transcribed from the respective DNA codons $\textrm{TAG}$, $\textrm{TGA}$, and $\textrm{TAA}$.