The Collaborative International Dictionary of English v.0.48:

Incubus \In"cu*bus\, n.; pl. E. Incubuses, L. Incubi. [L.,
   the nightmare. Cf. Incubate.]
   [1913 Webster]
   1. A demon; a fiend; a lascivious spirit, supposed to have
      sexual intercourse with women by night. --Tylor.
      [1913 Webster]

            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.
      [1913 Webster]

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

            Such as are troubled with incubus, or witch-ridden,
            as we call it.                        --Burton.
      [1913 Webster]

   3. Any oppressive encumbrance or burden; anything that
      prevents the free use of the faculties.
      [1913 Webster]

            Debt and usury is the incubus which weighs most
            heavily on the agricultural resources of Turkey.
                                                  --J. L.
                                                  Farley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

incubus
    n 1: a male demon believed to lie on sleeping persons and to
         have sexual intercourse with sleeping women
    2: a situation resembling a terrifying dream [syn: nightmare,
       incubus]
    3: someone who depresses or worries others

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "incubus":
   Baba Yaga, Dracula, Frankenstein, Lilith, Wolf-man, afreet,
   bad dream, bale, barghest, bogey, bogeyman, brown study, bugaboo,
   bugbear, burden, burdening, burthen, cacodemon, cargo, charge,
   charging, cumber, cumbrance, daeva, daydream, deadweight, demon,
   devil, drag, dream, dybbuk, evil spirit, fee-faw-fum, fiend,
   fiend from hell, freight, frightener, genie, genius, ghost, ghoul,
   gyre, handicap, hellion, hobgoblin, holy terror, horror,
   incumbency, jinni, jinniyeh, lading, lamia, load, loading,
   millstone, monster, nightmare, ogre, ogress, oppression, overload,
   overtaxing, overweighting, phantom, pipe dream, pressure, rakshasa,
   revenant, reverie, saddling, satan, scarebabe, scarecrow, scarer,
   shedu, specter, succubus, superincumbency, surcharge, taxing,
   terror, the undead, vampire, vision, werewolf, yogini



The Devil's Dictionary (1881-1906):

INCUBUS, n.  One of a race of highly improper demons who, though
probably not wholly extinct, may be said to have seen their best
nights.  For a complete account of _incubi_ and _succubi_, including
_incubae_ and _succubae_, see the _Liber Demonorum_ of Protassus
(Paris, 1328), which contains much curious information that would be
out of place in a dictionary intended as a text-book for the public
schools.
    Victor Hugo relates that in the Channel Islands Satan himself --
tempted more than elsewhere by the beauty of the women, doubtless --
sometimes plays at _incubus_, greatly to the inconvenience and alarm
of the good dames who wish to be loyal to their marriage vows,
generally speaking.  A certain lady applied to the parish priest to
learn how they might, in the dark, distinguish the hardy intruder from
their husbands.  The holy man said they must feel his brown for horns;
but Hugo is ungallant enough to hint a doubt of the efficacy of the
test.