A nightmare is a terrifying experience connected with the unpleasant emotions that awaken you, such as anxiety or fear.
A nightmare is often just a bad dream — isolated and disturbing imagery with no emotional reaction from the dreamer. A nightmare, however, can lead to feelings of fear, panic, and anxiety, to awaken the person and cause an upsetting emotional reaction, including insomnia, other sleep cycle difficulties, or even daytime discomfort. Fever, ill health, or poor nutrition can bring nightmares too.