retesting of a previously tested program following modification to ensure that faults have not been introduced or uncovered as a result of the changes made, and that the modified system still meets its requirements. It is performed whenever the software or its environment have changed.
BS7925-1
In other words, running the test scripts that succeeded before to see if they still do.
Usually contrasted with retesting, which is running the scripts whose failure pointed you toward the bugs in the first place.