Definition of "Halcyon" :

noun: 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

noun: a large kingfisher widely distributed in warmer parts of the Old World

noun: (Greek mythology) a woman who was turned into a kingfisher

adjective: marked by peace and prosperity

"The halcyon days of the clipper trade."

adjective: idyllically calm and peaceful; suggesting happy tranquillity

"A halcyon atmosphere."