A mutation that precisely restores a mutant DNA sequence to the wild-type DNA sequence.
For example -

• Base substitution mutations:
  

    UGG -> UGA  -> UGG
  
    trp -> stop -> trp
  
  

• Frameshift mutations:
  

    UGG GGG AAA ... -> UGG GGG GAA A.. -> UGG GGG AAA ...
  
    trp gly lys ...    trp gly glu ...    trp gly lys ...
  

• Insertion mutations: precise deletion ("excision") of the added DNA sequence

• Deletion mutations: cannot be restored by true reversion