Regression testing is any type of software testing which seeks to uncover regression bugs. Regression bugs occur whenever software functionality that previously worked as desired, stops working or no longer works in the same way that was previously planned. Typically regression bugs occur as an unintended consequence of program changes.
After modifying software, either for a change in functionality or to fix defects, a regression test re-runs previously passing tests on the modified software to ensure that the modifications haven’t unintentionally caused a regression of previous functionality. These regression tests are often automated.
More specific forms of regression testing are known as sanity testing, when quickly checking for erratic behavior, and smoke testing when testing for basic functionality.