revertant mutation – this term is used in two ways.
Firstly when referring to a genetic change that shows the virus is returning from a drug resistant mutation back to a wild-type genotype. This can sometimes take several stages. For example, T215E/D/E and S are revertant mutations that can indicate the drug resistant mutation T215Y is being replaced by wild-type (i.e. T215T).
It can also refer to the fitness of a virus. A revertant mutation can refer to an additional mutation that allows the virus to regain viral fitness (most drug mutations also make the vurs less fit). The second example is called a compensatory mutation as is compensates for the reduced fitness caused by the first mutation.