nightmare
n//ˈnaɪt.mɛə//mardröm, mara
A very unpleasant or frightening dream. [from 19th c.]
(figuratively) Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure. [from 20th c.]
(now, chiefly, historical) A feeling of extreme anxiety or suffocation experienced during sleep; sleep paralysis. [from 16th c.]
(now, rare) A demon or monster, thought to plague people while they slept and cause a feeling of suffocation and terror during sleep. [from 14th c.]