Or for the British out there a full stop. Or from the linked page:

"A nonsense mutation is a point mutation that introduces a premature stop codon into the part of the gene that encodes a protein. A stop codon is like a period at the end of a sentence: it instructs the ribosome to stop making the protein. So if a mutation leads to an early stop codon, only part of the protein will be made. 'Half-baked' proteins that result from nonsense mutations are often nonfunctional or defective."