A nightmare is a dream which causes a strong unpleasant emotional response from the sleeper, typically fear or horror, or the sensations of pain, falling, drowning or death. Such dreams can be related to physical causes such as a high fever, 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.

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).

...nightmare is the fourth release for the Italian hardcore punk band Negazione.