crucial has 5 distinct synonyms spread across 2 of its 3 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 noncrucial.
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.
of extreme importance; vital to the resolution of a crisis
“a crucial moment in his career”
of the greatest importance
“the all-important subject of disarmament”
1 of the 3 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 crucial.
Synonyms, antonyms and the broader and narrower relations for crucial come from Princeton University’s WordNet. Full sources and licences.