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.
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The Collaborative International Dictionary of English v.0.48:

Diver \Div"er\, n.
   1. One who, or that which, dives.
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            Divers and fishers for pearls.        --Woodward.
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   2. Fig.: One who goes deeply into a subject, study, or
      business. "A diver into causes." --Sir H. Wotton.
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   3. (Zool.) Any bird of certain genera, as Urinator
      (formerly Colymbus), or the allied genus Colymbus, or
      Podiceps, remarkable for their agility in diving.
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   Note: The northern diver (Urinator imber) is the loon; the
         black diver or velvet scoter (Oidemia fusca) is a sea
         duck. See Loon, and Scoter.
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