Regression refers to the reversion to an earlier stage of development in the face of unacceptable impulses. A reversion to immature patterns of behavior.When an individual retreats to an earlier stage of development that was both more secure and pleasant, this is referred to as regression.

A child hurt by a reprimand of the teacher may suck his thumb and cry, returning to a more secure and less mature time.

Other definition:

Regression means the returning to an earlier stage of psychosexual development under severely stressful conditions. The defense mechanism that occurs when extensive frustration causes a person to return to a stage that once provided a great deal of gratification. A statistical technique used for predicting one variable from another. The statistical process of finding the linear equation that produces the most accurate predicted values for Y using one predictor variable, X.