Succubus: A female demon supposed to descend upon and have sexual intercourse with a man while he sleeps.
In medieval legend, a 'succubus' is a female demon which comes to men, especially monks, in their dreams to seduce them and have sexual intercourse with them, drawing energy from the men to sustain themselves, often until the point of exhaustion or death. This legend was an explanation for the phenomena of wet dreams and sleep paralysis.
Incubus: An evil spirit supposed to descend upon and have sexual intercourse with women as they sleep.
It is a lascivious male demon said to possess mortal women as they sleep and to be responsible for the birth of demons, witches, and deformed children. According to one legend the incubus and his female counterpart, the succubus, were fallen angels. The belief in these demons was especially prevalent in the Middle Ages, and stories of assaults by incubi were not uncommon. There are similar spirits in many cultures. In current usage, incubus means a person or thing that oppresses, such as a nightmare.
The perceived origin of these demons, is said to be as an excuse, for women to have been cheating on their husbands. Saying that they were raped by a demon during the night. Back in those times, this was very well believable. If the woman got pregnant, she would still blame it on the Incubus.
More information for curious readers...
In medieval lore, a succubus was a female spirit that would lay with men to steal their seed: it was given as an explanation of night arousal and wet dreams. It was also paired with the incubus, the male form, sometimes transforming between the two forms to pass along the stolen seed to unknowing women.
In the fifteenth century, the succubus and the male demon, the counterpart incubus (which takes the form of a man, to seduce women), were associated with witchcraft, and witches were assumed to have intercourse with demons.
The Test: 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 brow for horns; but Hugo is ungallant enough to hint a doubt of the efficacy of the test.