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.