Generally, regression is a move backwards. Specifically:

• A term for the exploring, with the assistance of a therapist, of (possibly unpleasant) past memories (or some belief even past life, see past life regression ) as part of the treatment for mental illnesses. See psychotherapy, hypnotherapy.
• A re-introduction of a defect into a later revision of a product. See engineering, regression testing.
• The phenomenon by which the sea retreats, or regresses, as a consequence of lowering sea level. See geology.
• In statistics:
  • The phenomenon, that, when two related variables are measured, the expected value of the second is closer to the mean than the measured value of the first. See regression toward the mean.
  • A method where the mean of one or more random variables is predicted conditioned on other (measured) random variables. See regression analysis.