A regression is when a part of your code that once functioned correctly stops doing so. Most often, that is a result of changes made elsewhere in the code undermining the assumptions of the now-buggy section. When this happens, it's a good idea to add tests to your test suite that can recognize the bug. This ensures that, if you ever make a similar mistake again, the test suite will catch it immediately.

Tests that make sure that the working code doesn't become buggy are called regression tests. They can be written before or after a bug is found, and they provide you with the assurance that your program's complexity is not causing the bugs to multiply. Once your code passes a unit test, integration test, or a system test, you don't need to delete these tests from the test suite. You can leave them in place, and they will function as additional regression tests, letting you know if the test stops working.