What does halcyon mean?

adjective:
denoting a period of time in the past that was idyllically happy and peaceful.

synonyms:
serene, calm, pleasant, thriving, prosperous, successful - “the halcyon days of the 1960s”

antonyms:
stormy, troubled

noun:
1. a mythical bird said by ancient writers to breed in a nest floating at sea at the winter solstice, charming the wind and waves into calm.
2. a tropical Asian and African kingfisher with brightly coloured plumage.