regress has 15 distinct synonyms spread across 5 of its 6 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. Its one recorded opposite is progress.
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.
get worse or fall back to a previous condition
the reasoning involved when you assume the conclusion is true and reason backward to the evidence
returning to a former state
Synonyms, antonyms and the broader and narrower relations for regress come from Princeton University’s WordNet. Full sources and licences.