halcyon has 4 distinct synonyms spread across 3 of its 5 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.
(Greek mythology) a woman who was turned into a kingfisher
a large kingfisher widely distributed in warmer parts of the Old World
a mythical bird said to breed at the time of the winter solstice in a nest floating on the sea and to have the power of calming the winds and waves
1 of the 5 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 halcyon.
Synonyms, antonyms and the broader and narrower relations for halcyon come from Princeton University’s WordNet. Full sources and licences.