retroflex has 3 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.
articulate (a consonant) with the tongue curled back against the palate
“Indian accents can be characterized by the fact that speakers retroflex their consonants”
Synonyms, antonyms and the broader and narrower relations for retroflex come from Princeton University’s WordNet. Full sources and licences.
bent or curved backward
pronounced with the tip of the tongue turned back toward the hard palate