The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A very bitter compound of quassia, cocculus Indicus, etc.,
      used by fraudulent brewers in adulterating beer. --Cooley.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bittern \Bit"tern\, n. [OE. bitoure, betore, bitter, fr. F.
   butor; of unknown origin.] (Zool.)
   A wading bird of the genus Botaurus, allied to the herons,
   of various species.
   [1913 Webster]

   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 Botaurus lentiginosus, and is also called
         stake-driver and meadow hen. See Stake-driver.
         [1913 Webster]

   Note: The name is applied to other related birds, as the
         least bittern (Ardetta exilis), and the sun
         bittern.
         [1913 Webster]

WordNet (r) 3.0 (2006):

bittern
    n 1: relatively small compact tawny-brown heron with nocturnal
         habits and a booming cry; found in marshes

Easton's 1897 Bible Dictionary:

Bittern
   is found three times in connection with the desolations to come
   upon Babylon, Idumea, and Nineveh (Isa. 14:23; 34:11; Zeph.
   2:14). This bird belongs to the class of cranes. Its scientific
   name is Botaurus stellaris. It is a solitary bird, frequenting
   marshy ground. The Hebrew word (kippod) thus rendered in the
   Authorized Version is rendered "porcupine" in the Revised
   Version. But in the passages noted the kippod is associated with
   birds, with pools of water, and with solitude and desolation.
   This favours the idea that not the "porcupine" but the "bittern"
   is really intended by the word.