The Collaborative International Dictionary of English v.0.48:

dybbuk \dyb"buk\ (d[i^]b"b[u^]k; Hebrew d[=e]*b[=oo]k"), n.; pl.
   dybbuks; Hebr. dybbukim (d[=e]`b[=oo]k*[=e]m"). (Jewish
   folklore)
   the wandering soul of a dead person, or a demon, that enters
   the body of a living person and controls that body's
   behavior. It may be exorcised by religious rites.

   Syn: dibbuk.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

dybbuk
    n 1: (Jewish folklore) a demon that enters the body of a living
         person and controls that body's behavior [syn: dybbuk,
         dibbuk]