venture has 8 distinct synonyms spread across 4 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.
any venturesome undertaking especially one with an uncertain outcome
an investment that is very risky but could yield great profits
“he knew the stock was a speculation when he bought it”
a commercial undertaking that risks a loss but promises a profit
put at risk
“I will stake my good reputation for this”
Synonyms, antonyms and the broader and narrower relations for venture come from Princeton University’s WordNet. Full sources and licences.