Which testing type ensures that existing functionality is not adversely affected by new changes?

Regression testing is the type of testing specifically designed to ensure that existing functionality remains unaffected by new changes, such as enhancements, fixes, or any modifications to the codebase. The purpose of regression testing is to identify any unintended consequences of these changes that may impact existing features or performance. This is particularly important in Agile environments, where continuous integration and frequent updates are common.

By running regression tests after changes are introduced, testers can confidently ascertain that the software's existing features still function as expected and that new code integrations have not introduced faults. This type of testing is comprehensive and generally includes previously executed test cases to verify previously fixed issues or valid functionalities.

Acceptance testing focuses on whether the application meets the business requirements from the user's perspective rather than verifying the stability of existing features post-modifications. Functional testing assesses specific functionalities based on defined requirements but does not systematically ensure that older features work as intended after changes. Smoke testing, on the other hand, is a preliminary check to determine if the basic functionalities of the application are working before further testing is conducted, but it is not as thorough as regression testing in assessing the effects of new changes on existing functionalities.