nightmare

1. (figuratively) Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure.Examples: "Cleaning up after identity theft can be a nightmare of phone calls and letters."; "If Euston is not typically English, St. Pancras is. Its façade is a nightmare of improbable Gothic. It is fairly plastered with the aesthetic ideals of 1868, and the only beautiful thing about it is Barlow's roof. It is haunted by the stuffier kind of ghost. Yet there is something about the ordered whole of St. Pancras that would make demolition a terrible pity."; "The Red Holocaust is best interpreted in this light as the bitter fruit of an^([sic]) utopian gambit that was socially misengineered into a dystopic nightmare by despots in humanitarian disguise."figuratively

2. (now rare) A demon or monster, thought to plague people while they slept and cause a feeling of suffocation and terror during sleep.Examples: "It haunted me, however, more than once, like a night-mare."; "I started, hourly, from dreams of unutterable fear, to find the hot breath of the thing upon my face, and its vast weight—an incarnate Night-Mare that I had no power to shake off—incumbent eternally upon my heart!"Synonyms: incubus, succubus, night hag, sleep paralysisarchaic

3. (now chiefly historical) A feeling of extreme anxiety or suffocation experienced during sleep; sleep paralysis.Examples: "The Night-mare generally ſeizes people ſleeping on their backs, and often begins with frightful dreams, which are ſoon ſucceeded by a difficult reſpiration, a violent oppreſſion on the breaſt, and a total privation of voluntary motion."; "Had been afflicted in the night with that strange complaint called the nightmare."historical