Source : Webster's Revised Unabridged Dictionary (1913)

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}.

   2. (Med.) The nightmare. See {Nightmare}, 2.

Source : WordNet®

succubus
     n : a female demon believed to have sexual intercourse with
         sleeping men [syn: {succuba}]
     [also: {succubi} (pl)]