Answer : Maintenance testing
The purpose of regression testing is to confirm that a recent program or code change has not adversely affected existing features.
Regression testing is nothing but full or partial selection of already executed test cases which are re-executed to ensure existing functionalities work fine.
This testing is done to make sure that new code changes should not have side effects on the existing functionalities. It ensures that old code still works once the new code changes are done.
Need of Regression Testing
Regression Testing is required when there is a
Change in requirements and code is modified according to the requirement
New feature is added to the software
Performance issue fix
Reference : Regression Testing