a mutation of a gene caused by the addition or deletion of any number of nucleotides other than three or multiples of three in a DNA sequence: messenger RNA derived from such DNA will carry faulty instructions for the synthesis of proteins
See frameshift mutation in American Heritage Dictionary 4
noun
A mutation in a DNA chain that occurs when the number of nucleotides inserted or deleted is not a multiple of three, so that every codon beyond the point of insertion or deletion is read incorrectly during translation.