WordNet records no synonym and no antonym for postposition. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
(linguistics) the placing of one linguistic element after another (as placing a modifier after the word that it modifies in a sentence or placing an affix after the base to which it is attached)
Synonyms, antonyms and the broader and narrower relations for postposition come from Princeton University’s WordNet. Full sources and licences.