A nightmare, also called a bad dream, is an unpleasant dream that can cause a strong emotional response from the mind, typically fear but also despair, anxiety and great sadness.
Nightmares are lengthy, elaborate dreams with imagery that evokes fear, anxiety, or sadness. The dreamer may wake up to avoid the perceived danger. Nightmares can be remembered upon awakening and may lead to difficulties returning to sleep or even cause daytime distress.