A very upsetting or frightening dream: An extremely unpleasant event or experience or…. Learn more.

The dream may … A terrifying dream in which the dreamer experiences feelings of helplessness, extreme anxiety, sorrow, etc. See examples of nightmare used in a sentence. It was not until the 16th century that the word nightmare came to mean a frightening dream. this meaning probably came from the belief that such dreams were caused by evil spirits.