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