Mutational change in a codon that does not result in a change in the specified amino acid.
Synonymous substitutions can be though are not necessarily silent mutations. That is, these are mutations that result in no change in , though they could impact organism physiology and therefore fitness by modifying the rate of synthesis of the resulting protein. See, that is, the concept of .
Notwithstanding this caveat, a synonymous substitution generally is assumed to be more silent than a nonsynonymous substitution. A synonymous substitution to a first approximation therefore is assumed to at least approximate a neutral mutation. The fixation of such a mutation is presumed consequently to be a consequence of some manifestation of genetic drift. See, that is, the concept of .
Consider too the concept of dN/dS as well as redundancy of the genetic code along with that of wobble.