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]