Regression Testing verifies that anything unchanged still performs correctly. For example re-running of previously conducted tests to ensure that the unchanged system segments function properly.
Regression testing should be used when there is a high risk that new changes may affect unchanged areas of the application system. Software development organizations with effective regression testing policies significantly improve the effectiveness of their software development staff and the success of their projects.
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The goal of regression testing is to assure that things that worked correctly in the previous build still work with the newly added code and that there are no side effects. [Source]
Also See:
Difference between Regression & Re-testing