Learn in the textbookValidation, Acceptance, and Migration
DefinitionRegression testing (リグレッションテスト, also 回帰テスト) rechecks, after a program is changed, that existing features that were not changed still work as before.
ExplanationA fix or a new feature can break some other feature in an unexpected place. So apart from testing the changed part, you rerun the tests that passed before and see whether any result has changed.
AnalogyIt is like getting your car's brakes repaired and then trying not only the brakes but also the lights and wipers to see that they still work as before.
Memory hookTo regress means to go backward, so the name itself reminds you that this test looks for quality slipping backward because of a change.