The Collaborative International Dictionary of English v.0.48:

Beluga \Be*lu"ga\ (b[-e]*l[=u]"g[.a]), n. [Russ. bieluga a sort
   of large sturgeon, prop. white fish, fr. bieluii white. The
   whale is now commonly called bieluka in Russian.]
   1. (Zool.) A cetacean allied to the dolphins.
      [1913 Webster]

   Note: The northern beluga (Delphinapterus catodon) is the
         white whale and white fish of the whalers. It grows
         to be from twelve to eighteen feet long.
         [1913 Webster]

   2. the sturgeon (Huso huso) native to the Black Sea and
      Caspian Sea; -- also called hausen. It is valued for its
      roe, sold as caviar, and is also used for production of
      isinglass. See also sturgeon.
      [PJC]

   3. the caviar obtained from the beluga[2]; -- also called
      beluga caviar. The caviar of the beluga is considered
      the finest sort, larger and of a taste superior to that
      obtained from other sturgeon. See also sturgeon and
      caviar.
      [PJC]