The Collaborative International Dictionary of English v.0.48:

Loon \Loon\, n. [For older loom, Icel. l[=o]mr; akin to Dan. &
   Sw. lom.] (Zool.)
   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 (Urinator
   septentrionalis), are the best known species. See Diver.
   [1913 Webster]