threat has 3 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 is a source of danger
“earthquakes are a constant threat in Japan”
a warning that something unpleasant is imminent
“they were under threat of arrest”
Synonyms, antonyms and the broader and narrower relations for threat come from Princeton University’s WordNet. Full sources and licences.
declaration of an intention or a determination to inflict harm on another
“his threat to kill me was quite explicit”