Source : Webster's Revised Unabridged Dictionary (1913)

Loon \Loon\, n. [For older loom, Icel. l?mr; akin to Dan. & Sw.
   lom.] (Zo["o]l.)
   Any one of several aquatic, wed-footed, northern birds of the
   genus {Urinator} (formerly {Colymbus}), noted for their
   expertness in diving and swimming under water. The common
   loon, or great northern diver ({Urinator imber}, or {Colymbus
   torquatus}), and the red-throated loon or diver ({U.
   septentrionalis}), are the best known species. See {Diver}.