a terrifying or deeply upsetting dream
"After waking from that nightmare, she refused to close her eyes again until sunrise."
A demon or monster, thought to plague people while they slept and cause a feeling of suffocation and terror during sleep.
"Ancient tales speak of a nightmare that presses down on the sleeper's chest, stealing their breath until they wake in cold sweat."
In plain English: A nightmare is a scary dream that makes you wake up feeling afraid.
"The loud noise kept me awake all night and turned my sleep into a nightmare."
Usage: Use nightmare to describe terrifying dreams that often involve being chased or unable to breathe, rather than literal monsters in the waking world. While it can metaphorically refer to any extremely bad situation, reserve this specific meaning for contexts involving actual sleep disturbances.
The word "nightmare" comes from Old English nihtmare, combining the words for night and mare, which was an evil spirit believed to afflict sleeping people. This term traveled into modern English through Middle English while retaining its original meaning of a supernatural creature causing nightmares.