<p><strong>Noun</strong></p><ul><li>a terrifying dream in which the dreamer experiences feelings of helplessness, extreme anxiety, sorrow, etc.</li><li>Synonyms: phantasmagoria</li><li>a condition, thought, or experience suggestive of a nightmare: the nightmare of his years in prison.</li><li>(formerly) a monster or evil spirit believed to oppress persons during sleep.</li></ul>