1911 Encyclopædia Britannica/Incubus

INCUBUS (a Late Latin form of the classical incubo, a night-mare, from incubare, to lie upon, weigh down, brood), the name given in the middle ages to a male demon which was supposed to haunt women in their sleep, and to whose visits the birth of witches and demons was attributed. The female counterparts of these demons were called succubae. The word is also applied generally to an oppressive thing or person.