What is regression testing?

Regression testing checks that recent code, configuration, or dependency changes did not break existing behavior. Teams usually re-run a stable set of tests - often automated UI or API flows - after each merge, release candidate, or hotfix. The goal is to catch unintended side effects early, not to explore new features. Good regression suites focus on high-risk paths: login, checkout, billing, permissions, and other revenue- or security-critical flows. Suites grow over time; prioritization and maintenance matter as much as coverage. Regression testing complements exploratory testing and does not replace it.