n.  1.  The act of passing back or returning; retrogression; retrogradation.

Noun  1.  regression  an abnormal state in which development has stopped prematurely 
2.  regression  (psychiatry) a defense mechanism in which you flee from reality by assuming a more infantile state  
3.  regression  the relation between selected values of x and observed values of y (from which the most probable value of y can be predicted for any value of x)  
4.  regression  returning to a former state 
1.  (mathematics)  regression  A mathematical method where an empirical function is derived from a set of experimental data.  
2.  regression  regression testing. 