Two forms of the same folkloric idea, distinguished by position and by which sex they visit. A succubus is a female demon said to lie with sleeping men; an incubus is a male one said to lie with sleeping women. The Latin names describe under and on top.
The core difference
- succubus — female demon, from Latin succubare, to lie beneath.
- incubus — male demon, from incubare, to lie upon.
The names are anatomically literal, which is unusually blunt for a piece of medieval demonology.
Where the belief came from
Both are widely understood as folk explanations for sleep paralysis — a state in which a person wakes unable to move, often with a sensation of pressure on the chest and a vivid feeling of a presence in the room. It is well documented physiologically, and it occurs across every culture, which is why nearly every culture has a name for the presence.
- Mare — the Germanic spirit that sits on a sleeper's chest, and the second half of nightmare.
- Kanashibari — Japanese, literally bound in metal.
- Old Hag — Newfoundland English.
- Pisadeira — Brazilian, a figure who treads on the chest.
The medieval theological problem
Church writers took the question seriously enough to argue about mechanism, since demons were held to have no generative capacity of their own. One proposed solution had a single demon act as succubus and then as incubus in order to transfer material — which is why some accounts insist the two are the same entity in different forms rather than two kinds of being.
Nightmare has nothing to do with horses
The -mare is the Old English mære, that chest-sitting spirit. The horse sense of mare is a separate word entirely, from Old English mere. The two converged in spelling and have confused people ever since — Fuseli's famous painting The Nightmare includes a horse, which is a visual pun rather than a reference.
The plurals
Succubi and incubi, following Latin second-declension nouns — the same pattern as cactus and cacti, or fungus and fungi. Succubuses and incubuses are also attested and unobjectionable in English.
Frequently asked questions
What is the difference between a succubus and an incubus?
A succubus is a female demon said to visit sleeping men; an incubus is a male one said to visit sleeping women. The Latin names mean to lie beneath and to lie upon.
Are they explanations for sleep paralysis?
They are widely understood that way. Sleep paralysis produces immobility, chest pressure and a sensed presence, and nearly every culture has a name for that figure.
Is nightmare related to horses?
No. The -mare is an Old English word for a spirit that sits on a sleeper's chest. The horse sense is a separate word that converged in spelling.