The Free On-line Dictionary of Computing (30 December 2018):

regression testing

    Part of the test phase of software development
   where, as new modules are integrated into the system and the
   added functionality is tested, previously tested functionality
   is re-tested to assure that no new module has corrupted the
   system.

   [Bennatan, E.M., "Software Project Management", 2nd edition,
   McGraw-Hill International, 1992].

   (1995-12-04)