A silent mutation is usually a substitution mutation that causes a change in a codon that codes for the same amino acid as the initial codon.
Explanation:
This is possible because just 20 amino acids are coded for by 64 codons. An example is Leucine that is coded by 4 codons; GAA, GAG, GAT, and GAC. If you look closely, a substitution mutation of the last nucleotide of the codon will less probably cause a change in the translated amino acid. These mutation are therefore non-lethal.
