Source : Webster's Revised Unabridged Dictionary (1913)

Incubus \In"cu*bus\, n.; pl. E. {Incubuses}, L. {Incubi}. [L.,
   the nightmare. Cf. {Incubate}.]
   1. A demon; a fiend; a lascivious spirit, supposed to have
      sexual intercourse with women by night. --Tylor.

            The devils who appeared in the female form were
            generally called succubi; those who appeared like
            men incubi, though this distinction was not always
            preserved.                            --Lecky.

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

            Such as are troubled with incubus, or witch-ridden,
            as we call it.                        --Burton.

   3. Any oppressive encumbrance or burden; anything that
      prevents the free use of the faculties.

            Debt and usury is the incubus which weighs most
            heavily on the agricultural resources of Turkey.
                                                  --J. L.
                                                  Farley.