An incubus is a demon in male form who, according to mythological and legendary traditions, lies upon sleepers, especially women, in order to engage in sexual activity with them. Its female counterpart is the succubus. An incubus may pursue sexual relations with a woman in order to father a child, as in the legend of Merlin. Religious tradition holds that repeated sexual activity with an incubus or succubus may result in the deterioration of health, or even death.

The word incubus is derived from Late Latinincubo (a nightmare induced by such a demon); from incub(āre) (to lie upon). One of the earliest mentions of an incubus comes from Mesopotamia on the Sumerian King List, ca. 2400 BC, where the hero Gilgamesh's father is listed as Lilu. It is said that Lilu disturbs and seduces women in their sleep, while Lilitu, a female demon, appears to men in their erotic dreams. Two other corresponding demons appear as well: Ardat lili, who visits men by night and begets ghostly children from them, and Irdu lili, who is known as a male counterpart to Ardat lili and visits women by night and begets from them. These demons were originally storm demons, but they eventually became regarded as night demons because of mistaken etymology.

Debate about the demons began early in the Christian tradition. St. Augustine touched on the topic in De Civitate Dei ("The City of God"). There were too many attacks by incubi to deny them. He stated, "There is also a very general rumor. Many have verified it by their own experience and trustworthy persons have corroborated the experience others told, that sylvans and fauns, commonly called incubi, have often made wicked assaults upon women. Questions about the reproductive capabilities of the demons continued. Eight hundred years later, Thomas Aquinas lent himself to the ongoing discussion, stating, Still, if some are occasionally begotten from demons, it is not from the seed of such demons, nor from their assumed bodies, but from the seed of men, taken for the purpose; as when the demon assumes first the form of a woman, and afterwards of a man; just so they take the seed of other things for other generating purposes. It became generally accepted that incubi and succubi were the same demon, able to switch between male and female forms. A succubus would be able to sleep with a man and collect his sperm, and then transform into an incubus and use that seed on women. Even though sperm and egg came from humans originally, the spirits' offspring were often thought of as supernatural.

Though many tales claim that the incubus is bisexual, others indicate that it is strictly heterosexual and finds attacking a male victim either unpleasant or detrimental.

Incubi are sometimes said to be able to conceive children. The half-human offspring of such a union is sometimes referred to as acambion. The most famous legend of such a case includes that of Merlin, the famous wizard from Arthurian legend.

According to the Malleus Maleficarum, exorcism is one of the five ways to overcome the attacks of incubi, the others being Sacramental Confession, the Sign of the Cross or recital of the Angelic Salutation, moving the afflicted to another location, and by excommunication of the attacking entity, which is perhaps the same as exorcism. On the other hand, the Franciscan friar Ludovico Maria Sinistrari stated that incubi, do not obey exorcists, have no dread of exorcisms, show no reverence for holy things, at the approach of which they are not in the least overawed.