nightmare

noun

night·​mare ˈnīt-ˌmer 

1

: an evil spirit formerly thought to oppress people during sleep

2

: a frightening dream that usually awakens the sleeper

3

: something (such as an experience, situation, or object) having the monstrous character of a nightmare or producing a feeling of anxiety or terror

nightmare adjective

nightmarish

ˈnīt-ˌmer-ish 

adjective

nightmarishly adverb

Did you know?

Looking at nightmare, you might guess that it is a compound formed from night and mare. If so, your guess is correct. But while the night in nightmare makes sense, the mare part is less obvious. Most English speakers know mare as a word for a female horse or similar equine animal, but the mare of nightmare is a different word, an obsolete one referring to an evil spirit that was once thought to produce feelings of suffocation in people while they slept. By the 14th century the mare was also known as nightmare, and by the late 16th century nightmare was also being applied to the feelings of distress caused by the spirit, and then to frightening or unpleasant dreams.

Synonyms

• agony
• misery
• horror
• torture
• torment
• hell

Examples of nightmare in a Sentence

Mommy, I had a really scary nightmare.

The party was a complete nightmare.

Recent Examples on the Web

None of this is meant to prompt nightmares of seventh grade civics class, raise doubts about one’s ability to pass a citizenship test, or question the arrangements for Friday’s barbeque. —Michael Peregrine, Forbes.com, 1 July 2025

For centuries, folklore and popular wisdom have linked poor eating habits and indigestion to nightmares and restless sleep. —Jeffrey Kluger, Time, 1 July 2025

Best Overall Deals Flip-flops are seemingly every podiatrist’s nightmare, but these Vionic flip-flops are an exception. —Olivia Young, Travel + Leisure, 30 June 2025

These insects are a nightmare in our homes but are part of nature’s intricate workings outside them. —Madeline Bodin, Smithsonian Magazine, 27 June 2025

Word History

Etymology

Middle English nyghte mare, from nyghte night entry 1 + mare mare entry 3

First Known Use

14th century, in the meaning defined at sense 1

Phrases Containing nightmare

• worst nightmare

Rhymes for nightmare

• abwehr
• affair
• affaire
• airfare
• altair
• armchair
• aware
• beware
• bugbear
• carfare
• childcare
• compare

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Kids Definition

nightmare

noun

night·​mare ˈnīt-ˌma(ə)r 

-ˌme(ə)r

1

: a frightening dream

2

: a frightening or horrible experience

nightmarish

-ish 

adjective

Etymology

Middle English nightmare "evil spirit thought to haunt people during sleep," from night "night" and mare "spirit," from Old English mare "evil spirit haunting people in sleep"

Word Origin

In the Middle Ages many people believed in a type of evil spirit that was said to haunt people while they slept. Such a demon was known in Old English as a mare. Because it was believed to come in the night, it later became known in Middle English as a nightmare. 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.

Medical Definition

nightmare

noun

night·​mare ˈnīt-ˌma(ə)r, -ˌme(ə)r 

: a frightening or distressing dream that usually awakens the sleeper