ignis fatuus


ig·​nis fat·​u·​us ˈig-nəs-ˈfa-chə-wəs How to pronounce ignis fatuus (audio)
plural ignes fatui ˈig-ˌnēz-ˈfa-chə-ˌwī How to pronounce ignis fatuus (audio)
: a light that sometimes appears in the night over marshy ground and is often attributable to the combustion of gas from decomposed organic matter
: a deceptive goal or hope

Did you know?

Ignis fatuus is a Latin term meaning, literally, "foolish fire." Other names for this light are jack-o'-lantern and will-o'-the-wisp—both of which are connected to folklore about mysterious men, Jack and Will, who carry a lantern or a wisp of light at night. A Scottish name for ignis fatuus is spunkie, from spunk, meaning "spark" or "a small fire." It has also been told that ignes fatui (the Latin plural form) are roaming souls. No doubt these stories spooked listeners by candlelight, but in time, advancements in science not only gave us electricity to dispel the darkness but proved ignis fatuus to be a visible exhalation of gas from the ground, which is rarely seen today.

Examples of ignis fatuus in a Sentence

Recent Examples on the Web Several sources say ignis fatuus, a spark of swamp gas, is the likely cause of the strange light. Fox News, 3 Mar. 2023 The story also helped explain ignis fatuus, a natural phenomenon that occurs in marshlands and bogs—such as those in Ireland’s countryside—producing flickering lights as gases from decomposing organic matter combust. National Geographic, 27 Oct. 2020

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'ignis fatuus.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


Medieval Latin, literally, foolish fire

First Known Use

1563, in the meaning defined at sense 1

Time Traveler
The first known use of ignis fatuus was in 1563


Dictionary Entries Near ignis fatuus

Cite this Entry

“Ignis fatuus.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/ignis%20fatuus. Accessed 5 Dec. 2023.

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