assumption has 10 distinct synonyms spread across 5 of its 7 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 statement that is assumed to be true and from which a conclusion can be drawn
“on the assumption that he has been injured we can infer that he will not play”
a hypothesis that is taken for granted
“any society is built upon certain assumptions”
the act of taking possession of or power over something
“his assumption of office coincided with the trouble in Cuba”
celebration in the Roman Catholic Church of the Virgin Mary's being taken up into heaven when her earthly life ended; corresponds to the Dormition in the Eastern Orthodox Church
(Christianity) the taking up of the body and soul of the Virgin Mary when her earthly life had ended
audacious (even arrogant) behavior that you have no right to
“he despised them for their presumptuousness”
Synonyms, antonyms and the broader and narrower relations for assumption come from Princeton University’s WordNet. Full sources and licences.