Source : Webster's Revised Unabridged Dictionary (1913)

Bittern \Bit"tern\, n. [OE. bitoure, betore, bitter, fr. F.
   butor; of unknown origin.] (Zo["o]l.)
   A wading bird of the genus {Botaurus}, allied to the herons,
   of various species.

   Note: The common European bittern is {Botaurus stellaris}. It
         makes, during the brooding season, a noise called by
         Dryden bumping, and by Goldsmith booming. The American
         bittern is {B. lentiginosus}, and is also called
         {stake-driver} and {meadow hen}. See {Stake-driver}.

   Note: The name is applied to other related birds, as the
         {least bittern} ({Ardetta exilis}), and the {sun
         bittern}.

Bittern \Bit"tern\, n. [From {Bitter}, a.]
   1. The brine which remains in salt works after the salt is
      concreted, having a bitter taste from the chloride of
      magnesium which it contains.

   2. A very bitter compound of quassia, cocculus Indicus, etc.,
      used by fraudulent brewers in adulterating beer. --Cooley.

Source : WordNet®

bittern
     n : relatively small compact tawny-brown heron with nocturnal
         habits and a booming cry; found in marshes