Regression testing is a software testing process that verifies if recent code changes to an application have adversely affected its existing functionality. It involves re-running previously conducted tests on the modified software to ensure that any new changes have not introduced bugs or errors into the system.

The primary goal of regression testing is to identify and fix defects that may have been inadvertently introduced during the development of new features, bug fixes, or enhancements. It helps ensure the stability and reliability of the software by validating that existing features continue to work as expected after changes are made.

Regression testing typically involves the following steps:

1. Test selection: Identifying which existing test cases need to be re-executed based on the areas of the code that have been modified.

2. Test execution: Running the selected test cases to validate the functionality of the modified software.

3. Defect identification: Analyzing the test results to identify any failures or deviations from expected behavior.

4. Defect resolution: Investigating and fixing any defects that are discovered during regression testing.

5. Re-testing: Re-running the affected test cases after the defects have been fixed to ensure that the issues have been resolved and that the software still functions correctly.

Regression testing can be performed manually or automated, depending on the complexity of the software and the resources available. Automated regression testing involves using specialized tools to execute test scripts and compare actual results with expected results, significantly reducing the time and effort required for testing.