night·​mare ˈnīt-ˌmer How to pronounce nightmare (audio)
: an evil spirit formerly thought to oppress people during sleep
: a frightening dream that usually awakens the sleeper
: 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 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.

Example Sentences

Mommy, I had a really scary nightmare. The party was a complete nightmare.
Recent Examples on the Web What an entrepreneur thought may have been a great way to update their brand to something more modern and in alignment with their company’s values could end up being a nightmare of confused customers and lost sales. Expert Panel, Forbes, 8 Mar. 2023 However, none of these experiences could have prepared us for the nightmare of a full-scale invasion. Anna Konieczyńska, Vogue, 25 Feb. 2023 The bride and groom got married in Charlotte, North Carolina on Sunday and suffered the ultimate wedding-day nightmare of getting trapped in a hotel elevator on the way to their reception party on the 16th floor of the Grand Bohemian Hotel. Marisa Sullivan, Peoplemag, 23 Feb. 2023 No, because Elizabeth’s life has been pretty sheltered in certain ways up until that nightmare of a story. Brian Davids, The Hollywood Reporter, 16 Feb. 2023 But for the thousands of Ukrainians who either can’t leave or refuse to leave homes in the firing line, there is no end in sight to the waking nightmare of war. Richard Engel, NBC News, 11 Feb. 2023 Now after 33 years, Salman Rushdie is left with the nightmare of death that will never leave him. Ananya Bhattacharya, Quartz, 6 Feb. 2023 Wondery Scandals have shaped American history, from the ecological and corporate nightmare of the Exxon Valdez oil spill to the horrors of the Tuskegee Syphilis Study, and this podcast is dedicated to shining a light on them all. Lauren Hubbard, Town & Country, 30 Jan. 2023 The nightmare of a Big 12 slate continues, as the Sooners have now lost their fourth game by four points or less. Dallas News, 21 Jan. 2023 See More

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'nightmare.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

Word History


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

Time Traveler
The first known use of nightmare was in the 14th century


Dictionary Entries Near nightmare

Cite this Entry

“Nightmare.” Dictionary, Merriam-Webster, Accessed 25 Mar. 2023.

Kids Definition


night·​mare ˈnīt-ˌma(ə)r How to pronounce nightmare (audio)
: a frightening dream
: a frightening or horrible experience
nightmarish adjective

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"

Medical Definition


: a frightening or distressing dream that usually awakens the sleeper

More from Merriam-Webster on nightmare

Last Updated: - Updated example sentences
Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!

What Did You Just Call Me?

  • brown chihuahua sitting on the floor with squinting eyes looking at the camera
  • Before we went to her house, Hannah told us her aunt was a flibbertigibbet.
Name That Thing

You know what it looks like… but what is it called?

Solve today's spelling word game by finding as many words as you can with using just 7 letters. Longer words score more points.

Can you make 12 words with 7 letters?