proposition has 3 distinct synonyms spread across 2 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. 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.
(logic) a statement that affirms or denies something and is either true or false
an offer for a private bargain (especially a request for sexual favors)
the act of making a proposal
“they listened to her proposal”
a task to be dealt with
“securing adequate funding is a time-consuming proposition”
suggest sex to
“She was propositioned by a stranger at the party”
Synonyms, antonyms and the broader and narrower relations for proposition come from Princeton University’s WordNet. Full sources and licences.