From Software Testing perspective, retesting involves following two set of tasks:
1. Retesting the defect to confirm the issue associated with the defect resolved successfully (or simple to say Defect Retest)
2. Retesting the failed test cases associated with the defect
Note that retesting is different from regression testing. Scope of retesting limited to failed test cases associated with the defect while regression testing involves testing the similar/relevant functionality associated with the functionality under question (or having an issue/defect).