WordNet records no synonym and no antonym for preposition. It is the only member of each of its synsets, 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.
a function word that combines with a noun or pronoun or noun phrase to form a prepositional phrase that can have an adverbial or adjectival relation to some other word
(linguistics) the placing of one linguistic element before another (as placing a modifier before the word it modifies in a sentence or placing an affix before the base to which it is attached)
Synonyms, antonyms and the broader and narrower relations for preposition come from Princeton University’s WordNet. Full sources and licences.