decisive has 1 distinct synonym spread across 1 of its 4 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. Its one recorded opposite is indecisive.
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.
determining or having the power to determine an outcome
“cast the decisive vote”
characterized by decision and firmness
“an able and decisive young woman”
forming or having the nature of a turning point or crisis
“a critical point in the campaign”
1 of the 4 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for decisive.
Synonyms, antonyms and the broader and narrower relations for decisive come from Princeton University’s WordNet. Full sources and licences.