noun
a phosphorescent light that hovers or flits over swampy ground at night, possibly caused by the combustion of gases from decomposed organic matter
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The ignis fatuus is a natural phenomenon that can sometimes be mistaken for a ghostly light in the distance.
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I thought I saw an ignis fatuus while walking in the woods last night, but it turned out to be just a reflection.
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Don't be fooled by that ignis fatuus, it's just a trick of the light.
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His promises were like an ignis fatuus, leading me on but never delivering.