event has 7 distinct synonyms spread across 2 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. 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.
something that happens at a given place and time
a special set of circumstances
“in that event, the first possibility is excluded”
a phenomenon located at a single point in space-time; the fundamental observational entity in relativity theory
a phenomenon that follows and is caused by some previous phenomenon
“the magnetic effect was greater when the rod was lengthwise”
Synonyms, antonyms and the broader and narrower relations for event come from Princeton University’s WordNet. Full sources and licences.