A nightmare is a dream which causes a strong unpleasant emotional response from the sleeper, typically fear or horror, being in situations of extreme danger, or the sensations of pain, falling, drowning or death. Such dreams can be related to physical causes such as a high fever, turned faced down on a pillow during sleep (most often in the case of drowning nightmares), or psychological ones such as psychological trauma or stress in the sleeper's life, or can have no apparent cause. If a person has experienced a psychologically traumatic situation in life—for example, a person who may have been captured and tortured—the experience may come back to haunt them in their nightmares. Sleepers may waken in a state of distress and be unable to get back to sleep for some time. Eating before bed, which triggers an increase in the body's metabolism and brain activity, is another potential stimulus for nightmares .

Occasional nightmares are commonplace, but recurrent nightmares can interfere with sleep and may cause people to seek medical help. A recently proposed treatment consists of imagery rehearsal. This approach appears to reduce the effects of nightmares and other symptoms in acute stress disorder and post-traumatic stress disorder.

Practitioners of lucid dreaming claim that it can help conquer nightmares of this type, rather than of the traditional type (see below).

Historic use of term

Nightmare was the original term for the state later known as waking dream (cf. Mary Shelley and Frankenstein's Genesis), and more currently as sleep paralysis, associated with rapid eye movement (REM) sleep. The original definition was codified by Dr Johnson in his A Dictionary of the English Language and was thus understood, among others by Erasmus Darwin and Henry Fuseli, to include a "morbid oppression during sleep, resembling the pressure of weight upon the breast."

Such nightmares were widely considered to be the work of demons and more specifically incubi, which were thought to sit on the chests of sleepers. In Old English the name for these beings was mare or mære (from a proto-Germanic *marōn, related to Old High German and Old Norse mara), hence comes the mare part in nightmare. Etymologically cognate with Anglo-Saxon /mara/ ('incubus') may be Hellenic /Marōn/ (in the Odusseid) and Samskṛta /Māra/ (supernatural antagonist of the Buddha). (This "incubus" is more modernly referred to as sleep paralysis.)

Folk belief in Newfoundland, South Carolina and Georgia describe the negative figure of the Hag who leaves her physical body at night, and sits on the chest of her victim. The victim usually wakes with a feeling of terror, has difficulty breathing because of a perceived heavy invisible weight on his or her chest, and is unable to move i.e., experiences sleep paralysis. This nightmare experience is described as being "hag-ridden" in the Gullah lore. The "Old Hag" was a nightmare spirit in British and also Anglophone North American folklore.