conclusion has 13 distinct synonyms spread across 7 of its 9 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 position or opinion or judgment reached after consideration
“a decision unfavorable to the opposition”
an intuitive assumption
“jump to a conclusion”
the proposition arrived at by logical reasoning (such as the proposition that must follow from the major and minor premises of a syllogism)
a final settlement
“the conclusion of a business deal”
the act of making up your mind about something
“the burden of decision was his”
Synonyms, antonyms and the broader and narrower relations for conclusion come from Princeton University’s WordNet. Full sources and licences.