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}.

Source : WordNet®

least bittern
     n : small American bittern [syn: {Ixobrychus exilis}]