The Collaborative International Dictionary of English v.0.48:

Beelzebub \Be*el"ze*bub\, n.
   The title of a heathen deity to whom the Jews ascribed the
   sovereignty of the evil spirits; hence, the Devil or a devil.
   See Baal.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Beelzebub
    n 1: (Judeo-Christian and Islamic religions) chief spirit of
         evil and adversary of God; tempter of mankind; master of
         Hell [syn: Satan, Old Nick, Devil, Lucifer,
         Beelzebub, the Tempter, Prince of Darkness]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "Beelzebub":
   Abaddon, Aeshma, Angra Mainyu, Apollyon, Azazel, Belial, Eblis,
   Lilith, Lucifer, Mephisto, Mephistopheles, Old Nick, Old Scratch,
   Putana, Sammael, Satan, Shaitan, diablo, fiend, serpent



Easton's 1897 Bible Dictionary:

Beelzebub
   (Gr. form Beel'zebul), the name given to Satan, and found only
   in the New Testament (Matt. 10:25; 12:24, 27; Mark 3:22). It is
   probably the same as Baalzebub (q.v.), the god of Ekron, meaning
   "the lord of flies," or, as others think, "the lord of dung," or
   "the dung-god."


Hitchcock's Bible Names Dictionary (late 1800's):

Beelzebub, same as Baalzebub