The Collaborative International Dictionary of English v.0.48:

Succubus \Suc"cu*bus\, n.; pl. Succubi. [See Succuba.]
   1. A demon or fiend; especially, a lascivious spirit supposed
      to have sexual intercourse with the men by night; a
      succuba. Cf. Incubus.
      [1913 Webster]

   2. (Med.) The nightmare. See Nightmare, 2.
      [1913 Webster]

WordNet (r) 3.0 (2006):

succubus
    n 1: a female demon believed to have sexual intercourse with
         sleeping men [syn: succubus, succuba]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "succubus":
   Baba Yaga, Dracula, Frankenstein, Lilith, Satan, Wolf-man, afreet,
   barghest, bogey, bogeyman, bugaboo, bugbear, cacodemon, daeva,
   demon, devil, dybbuk, evil genius, evil spirit, fee-faw-fum, fiend,
   fiend from hell, frightener, genie, genius, ghost, ghoul, gyre,
   hellion, hobgoblin, holy terror, horror, incubus, jinni, jinniyeh,
   lamia, monster, nightmare, ogre, ogress, phantom, rakshasa,
   revenant, satan, scarebabe, scarecrow, scarer, shedu, specter,
   terror, the undead, vampire, werewolf, yogini