significant has 4 distinct synonyms spread across 3 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 2 recorded opposites are insignificant, nonsignificant.
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.
important in effect or meaning
“a significant change in tax laws”
too closely correlated to be attributed to chance and therefore indicating a systematic relation
“the interaction effect is significant at the .01 level”
Synonyms, antonyms and the broader and narrower relations for significant come from Princeton University’s WordNet. Full sources and licences.