WordNet sense 1 (an abnormal state in which development has stopped prematurely):

fixation, arrested development, infantile fixation

WordNet sense 2 (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)):

regression toward the mean, simple regression, statistical regression

WordNet sense 3 (returning to a former state):

regress, retrogression, reversion, retroversion, regress