Regression testing is a type of software testing that is performed to verify that changes made to an application or system have not introduced new defects or caused existing features to fail. It is a way to ensure that the software continues to function as expected after modifications have been made.

Regression testing involves re-running previously executed test cases on the modified code to ensure that all existing functionality continues to work correctly. This helps to identify any unintended consequences of changes, such as breaking existing functionality or introducing new bugs.

Regression testing is typically performed after major changes to an application, such as a new release, update, or bug fix. It can be performed manually or using automated testing tools, and can include both functional and non-functional testing, such as load testing and performance testing.

Overall, regression testing is an important aspect of software development and quality assurance, as it helps to ensure that changes to the software do not negatively impact existing functionality and that the application continues to meet the requirements and expectations of its users.