ignis fatuus

ig·nis fat·u·us  (gns fch-s)

n. pl. ig·nes fat·u·i (gnz fch-)

1. A phosphorescent light that hovers or flits over swampy ground at night, possibly caused by spontaneous combustion of gases emitted by rotting organic matter. Also called friar's lantern, jack-o'-lantern, will-o'-the-wisp, wisp.

2. Something that misleads or deludes; an illusion.

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[Medieval Latin : Latin ignis, fire + Latin fatuus, foolish.]

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ignis fatuus [ˈɪgnɪs ˈfætjʊəs]

n pl ignes fatui [ˈɪgniːz ˈfætjʊˌaɪ]

another name for will-o'-the-wisp

[from Medieval Latin, literally: foolish fire]

Thesaurus

Noun	1.	ignis fatuus - a pale light sometimes seen at night over marshy ground friar's lantern, jack-o'-lantern, will-o'-the-wisp light, visible light, visible radiation - (physics) electromagnetic radiation that can produce a visual sensation; "the light was filtered through a soft glass window"
	2.	ignis fatuus - an illusion that misleads will-o'-the-wisp fancy, phantasy, illusion, fantasy - something many people believe that is false; "they have the illusion that I am very wealthy"