regression has 10 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an abnormal state in which development has stopped prematurely
(psychiatry) a defense mechanism in which you flee from reality by assuming a more infantile state
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)
Synonyms, antonyms and the broader and narrower relations for regression come from Princeton University’s WordNet. Full sources and licences.