Other things can be halcyon, too, if they're peaceful, gentle, calm, or happy and carefree.
Well, it'd be nice, wouldn't it.
The first halcyon was a Greek demi-goddess called Alcyone. She was blissfully married to Ceyx, but in their besotted love for each other they would sometimes call each other by the names of the King and Queen of the gods, Zeus and Hera. Zeus wasn't flattered by this (he was notoriously grumpy) and one day when Ceyx was out fishing Zeus struck Ceyx's boat with a thunderbolt.
When Alcyone heard the news of Ceyx's loss she despairingly flung herself into the sea, but the gods in their compassion turned both Alcyone and Ceyx into halcyon birds so they could continue to live together. The gods also gave Alcyone and Ceyx the power to charm the seas to calmness during the time of year that they made their nest on the water (presumably this was so they didn't either lose their eggs or get seasick).
It's a beautiful story. Even better, there's a real-life halcyon:
It's otherwise known as the blue-breasted kingfisher, or Halcyon malimbica. (Photo by tj. haslam.)
Word To Use Today: halcyon. Halcyon comes from alkuōn, the Greek word for kingfisher. The kingfisher just might be called after Alcyone, but probably isn't.