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]