fortuity has 3 distinct synonyms spread across 1 of its 1 sense. 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.
anything that happens suddenly or by chance without an apparent cause
“winning the lottery was a happy accident”
Synonyms, antonyms and the broader and narrower relations for fortuity come from Princeton University’s WordNet. Full sources and licences.