The Collaborative International Dictionary of English v.0.48:

frameshift \frame"shift\, a. (Genetics)
   of, pertaining to, or causing a type of mutation consisting
   of the insertion or deletion of one or more nucleotides in
   the nucleic acid structure of a gene, when the number of base
   pairs inserted or deleted is not a multiple of three. If the
   addition or deletion occurs in multiples of three, the
   unaffected nucleotides in the genome remain in the proper
   order ("frame") to be correctly translated into protein; in
   such cases of insertions or deletions not causing a frame
   shift, a functional though altered protein may be produced by
   the organism. Frameshift mutations cause more profound
   changes in the composition of the protein resulting from
   translation of the mutated gene.
   [PJC]