noun

• (poetic) A kingfisher whose nesting by the sea was said, in classical mythology, to cause the Gods to restrain the wind and waves.
  "the Halcyon bird, or as some call it Alcedo or Kings-fisher, exceeds all mens conceit. […] Gods decree hath been, that all the watrie wildernesse should be quiet and made calme, without raine, wind, or tempest, during the time the Halcyon sitteth and bringeth forth her young-ones […]"
• (poetic) A kingfisher whose nesting by the sea was said, in classical mythology, to cause the Gods to restrain the wind and waves.
  "But novv hovv ſtands the vvind? / Into vvhat corner peeres my Halcions bill?"

adj

• (figurative) Calm, undisturbed, peaceful, serene.
  "Reflections of this kind may have trifling weight with men who hope to see realized in America the halcyon scenes of the poetic or fabulous age."