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]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "dybbuk":
   Baba Yaga, Lilith, Masan, afreet, apparition, appearance, astral,
   astral spirit, banshee, barghest, cacodemon, control, daeva, demon,
   departed spirit, devil, disembodied spirit, duppy, eidolon,
   evil genius, evil spirit, fiend, fiend from hell, form, genie,
   genius, ghost, ghoul, grateful dead, guide, gyre, hant, haunt,
   hellion, idolum, immateriality, incorporeal, incorporeal being,
   incorporeity, incubus, jinni, jinniyeh, lamia, larva, lemures,
   manes, materialization, oni, phantasm, phantasma, phantom,
   poltergeist, presence, rakshasa, revenant, satan, shade, shadow,
   shape, shedu, shrouded spirit, specter, spectral ghost, spirit,
   spook, sprite, succubus, the undead, theophany, unsubstantiality,
   vampire, vision, walking dead man, wandering soul, wraith, yogini,
   zombie