pathological mental process nightmare Dream that can cause a strong emotional response from the mind, typically fear or horror, but also despair, anxiety and great sadness. The dream may contain situations of danger, discomfort, psychological or physical terror. Nightmares can have physical causes such as sleeping in an uncomfortable or awkward position, having a fever, or psychological causes such as stress, anxiety, and ingestion of opioid drugs. http://en.wikipedia.org/wiki/Nightmare