The Collaborative International Dictionary of English v.0.48:

Halcyon \Hal"cy*on\ (h[a^]l"s[i^]*[o^]n), n. [L. halcyon,
   alcyon, Gr. "alkyw`n, 'alkyw`n: cf. F. halcyon.] (Zool.)
   A kingfisher. By modern ornithologists restricted to a genus
   including a limited number of species having omnivorous
   habits, as the sacred kingfisher (Halcyon sancta) of
   Australia.
   [1913 Webster]

         Amidst our arms as quiet you shall be
         As halcyons brooding on a winter sea.    --Dryden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Halcyon \Hal"cy*on\, a.
   1. Pertaining to, or resembling, the halcyon, which was
      anciently said to lay her eggs in nests on or near the sea
      during the calm weather about the winter solstice.
      [1913 Webster]

   2. Hence: Calm; quiet; peaceful; undisturbed; happy. "Deep,
      halcyon repose." --De Quincy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

halcyon
    adj 1: idyllically calm and peaceful; suggesting happy
           tranquillity; "a halcyon atmosphere"
    2: marked by peace and prosperity; "a golden era"; "the halcyon
       days of the clipper trade" [syn: golden, halcyon,
       prosperous]
    n 1: (Greek mythology) a woman who was turned into a kingfisher
         [syn: Alcyone, Halcyon]
    2: a large kingfisher widely distributed in warmer parts of the
       Old World [syn: Halcyon, genus Halcyon]
    3: 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

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "halcyon":
   Saturnian, at peace, at rest, balmy, bloodless, blooming,
   blossoming, booming, bright, calm, clear, cloistered, concordant,
   cool, dwindling, ebbing, even-tenored, exuberant, fair, fat,
   flourishing, flowering, fruiting, going strong, hushed, idyllic,
   impassive, in full swing, in good case, isolated, mild, moldering,
   orderly, pacific, palmy, pastoral, peaceable, peaceful, peacetime,
   piping, placid, prospering, quiescent, quiet, reposeful, reposing,
   restful, resting, rosy, secluded, sequestered, sequestrated,
   serene, sheltered, sleek, smooth, soft, still, still as death,
   stillish, stilly, stoic, stolid, subsiding, sunny, thriving,
   tranquil, unagitated, undisturbed, unmoved, unperturbed, unruffled,
   unstirring, untroubled, vigorous, waning