Regression testing is testing of an already tested program after modification to discover any defects introduced or uncovered as a result of changes in software or its environment. Regression testing ensures that previously working functionality still works after changes are made3 defines regression testing as follows:

Regression Testing is defined as a type of software testing 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.

I, III, IV, and V are incorrect statements about regression testing. Retesting of a fixed defect (I) is not regression testing, but confirmation testing or rework testing, which verifies that a specific defect has been resolved. Testing of new functionality in a program (III) is not regression testing, but functional testing or new feature testing, which verifies that new requirements are met by new code. Regression testing applies only to functional testing (IV) is not true, as regression testing can also apply to non-functional aspects such as performance or security. Tests that do not have to be repeatable, because they are only used once (V) are not regression testing, but exploratory testing or ad hoc testing, which are based on learning and discovery rather than predefined test cases.