Regression means to step backward. When software changes are made that break stuff that used to work, it is considered to regress. Regression is the opposite of progress.
"AsAlso as a consequence of the introduction of new bugs, program maintenance requires far more system testing per statement written than any other programming. Theoretically, after each fix one must run the entire batch of test cases previously run against the system, to ensure that it has not been damaged in an obscure way. In practice, such <i>regression testing</i> must indeed approximate this theoretical idea, and it is very costly." -- Fred Brooks, Mythical Man Month (p 122)
