In Greek mythology, Alcyone (also dubiously called Halcyone) and Ceyx were a married couple whose excessive romantic pride provoked the wrath of the god Zeus. Alcyone was a Thessalian princess, the daughter of King Aeolus of Aeolia. She married King Ceyx of Trachis, whose father was Eosphorus (the morning star), and the pair lived contentedly together.

Their sacrilegious behaviour — calling each other by the names of Zeus and Hera — incurred divine anger. While Ceyx travelled to consult an oracle, Zeus struck him dead with a thunderbolt. Morpheus, god of dreams, appeared to Alcyone in Ceyx's form to reveal his fate, and overwhelmed by grief she cast herself into the sea. Out of compassion the gods transformed both into kingfisher birds, the "halcyons," named after the wife.

Different ancient sources present variations: Ovid and Hyginus describe the tragedy as occurring during a shipwreck rather than as divine punishment for hubris, while Ovid uniquely adds the detail of Alcyone discovering her husband's corpse washed ashore before her suicide.

Alcyone was a Thessalian princess, the daughter of King Aeolus of Aeolia, either by Enarete or Aegiale. She was the sister of Salmoneus, Athamas, Sisyphus, Cretheus, Perieres, Deioneus, Magnes, Calyce, Canace, Pisidice and Perimede.

Alcyone later became queen of Trachis after marrying King Ceyx, the son of Eosphorus (often translated as Lucifer, the morning star). The couple were very happy together in Trachis.

According to Pseudo-Apollodorus's account, the couple often sacrilegiously called each other "Zeus" and "Hera." This angered Zeus, so while Ceyx was at sea — in order to consult an oracle, according to Ovid — the god killed him with a thunderbolt. Soon after, Morpheus, the god of dreams, disguised himself as Ceyx and appeared to Alcyone to tell her of her husband's fate. In her grief she threw herself into the sea, and out of compassion the gods changed them both into "halcyon birds" (common kingfishers), named after her.

Ovid and Hyginus both also recount the metamorphosis of the pair following Ceyx's loss in a terrible storm, though they omit the couple calling each other "Zeus" and "Hera" — and Zeus's resulting anger — as the reason for it.

Alkyóne comes from alkyón, which refers to a sea-bird with a mournful song or to a kingfisher bird in particular. The meaning of the word is uncertain, because alkyón is considered to be of pre-Greek, non-Indo-European origin. Folk etymology nonetheless related it to háls ("brine, sea, salt") and kyéo ("I conceive").

Kéyx, as the name of a sea-bird, appears to be related to kaúax, which is a ravenous sea-bird. These suggest that Kéyx may have been turned into either a sea mew or a tern.

Ovid and Hyginus both also make the metamorphosis the origin of the term "halcyon days," the days in winter when storms never occur. They state that these were originally the days each year during which Alcyone, as a kingfisher, made her nest on the beach and laid her eggs, while her father Aeolus, the god of the winds, helped her do so safely by restraining the winds and thus calming the waves.

Various kinds of kingfishers are named after the couple in reference to the metamorphosis myth: the genus Ceyx (river kingfishers) honours him, while the family Halcyonidae (tree kingfishers) and the genus Halcyon honour Alcyone, and the belted kingfisher carries the Latin species designation Megaceryle alcyon. The seagoing high-crested pterosaur species Alcione elainus of the Late Cretaceous is named for Alcyone.

Their story features in Geoffrey Chaucer's The Book of the Duchess and is the basis for the opera Alcyone by the French composer Marin Marais and a cantata Alcyone by Maurice Ravel. T.S. Eliot references the halcyon in The Dry Salvages, and the Canadian poet Archibald Lampman titled his final collection Alcyone (1899).