Halcyon n. : 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.

Halcyon a. : 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.

Halcyon a. : Hence: Calm; quiet; peaceful; undisturbed; happy.

Example:
Why would extreme sleeping pills like Halcyon only knock me out for about 4 and half hours?

Taxonomy
Halcyon, Dive Systems, Dealers, Distributors, history, the Comparison, colors, designs, styles, capacities, photographs, Lighting Systems, Recreational Diving, Technical Diving, BC, Surface Markers, Guideline, Dry Suit.

Defination from the web:
Manufacturer of Rebreathers and a scuba equipment company for the recreational and technical diver alike.