Meaning

noun an abnormal state in which development has stopped prematurely

noun (psychiatry) a defense mechanism in which you flee from reality by assuming a more infantile state

noun 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)

originLearned borrowing from Latin regressio. Equivalent to regress + -ion. The statistics sense comes from regression to the mean.

Etymology from Wiktionary, CC BY-SA 3.0