retroflexion has 2 distinct synonyms spread across 3 of its 3 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.
a turning or tilting backward of an organ or body part
“retroversion of the uterus”
an articulatory gesture made by turning the tip of the tongue back against the roof of the mouth
Synonyms, antonyms and the broader and narrower relations for retroflexion come from Princeton University’s WordNet. Full sources and licences.