Incubus (Lat.). Something more real and dangerous than the ordinary meaning given to the word, viz., that of “nightmare ”. An Incubus is the male Elemental, and Succuba the female, and these are undeniably the spooks of mediæval demonology, called forth from the invisible regions by human passion and lust. They are now called “Spirit brides” and “Spirit husbands” among some benighted Spiritists and spiritual mediums. But these poetical names do not prevent them in the least being that which they are—Ghools, Vampires and soulless Elementals; formless centres of Life, devoid of sense; in short, subjective protoplasms when left alone, but called into a definite being and form by the creative and diseased imagination of certain mortals. They were known under every clime as in every age, and the Hindus can tell more than one terrible tale of the dramas enacted in the life of young students and mystics by the Pisachas, their name in India.
Source: H.P.Blavatsky - The Theosophical Glossary