A single base pair substitution that prematurely codes for a stop in amino acid translation (stop codon)
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
A nonsense mutation is the substitution of a single base pair that leads to the appearance of a stop codon where previously there was a codon specifying an amino acid. The presence of this premature stop codon results in the production of a shortened, and likely nonfunctional, protein.