nightmare

1 of 2

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

nightmarish

2 of 2

adjective

night·​mar·​ish -ma(a)rish 

-mer-, 

-rēsh

: resembling or suggestive of a nightmare

had a nightmarish memory of certain doorknobs … horrible in color and in shape

—Royal Cortissoz

paints a nightmarish picture

—Henry Hazlitt

nightmarishly adverb

nightmarishness noun

plural -es

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 of nightmare

Noun

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

Examples of nightmare in a Sentence

Noun

Mommy, I had a really scary nightmare.

The party was a complete nightmare.

Recent Examples on the Web

Noun

The bench eventually became long and gifted with Te-Hina Paopao, Madina Okot and others, and then the Dream caused nightmares for their opponents with one of the grandest deals ever during WNBA free agency. — Terence Moore, Forbes.com, 21 June 2026

As the noise-rap-electro act Jane Remover shrieked and pleaded through a 90-minute marathon set at the Fonda on Thursday night, one very young couple dressed right out of a conservative‘s nightmare — gender-ambiguous, purple hair, facial piercings — tapped me on the shoulder. — Los Angeles Times, 19 June 2026

The grisly details were the stuff of nightmares. — Ryan Coleman, Entertainment Weekly, 19 June 2026

The killing scenes of the regime were so fresh in my nightmares when the war began and now the sound of airstrikes and fighter jets added to them. — Somayeh Malekian, ABC News, 18 June 2026

Word History

Etymology

Noun

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

First Known Use

Noun

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

Noun

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