ignis fatuus

n., pl., ig·nes fat·u·i (ĭg'nēz făch'ū-ī').

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, Also called will-o'-the-wisp, wisp.
2. Something that misleads or deludes; an illusion.

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

(FRY-uhrz LAN-tuhrn)

noun
A phosphorescent light seen over marshy ground at night, caused by spontaneous combustion of gases emitted by decomposing organic matter. A synonym is foxfire (not Firefox), especially for luminescence produced by fungi.

Etymology
The first use of the term is in John Milton's 1632 poem L'Allegro: "She was pinched and pulled, she said; / And he, by Friar's lantern led."]

Usage
"Question: What do you get when you cross a firefly with a tobacco plant. Answer: A cigarette that lights itself. The joke quickly made the rounds after a group of genetic engineers in California earlier this month announced that they had transferred into the cells of a tobacco plant the gene that causes a firefly to glow. The tobacco plant seems to rise out of the page like a will-o'-the-wisp or friar's lantern." — Chet Raymo; A Tale of a Firefly and a Tobacco Leaf; The Boston Globe; Nov 24, 1986.

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noun

An erroneous perception of reality: delusion, hallucination, illusion, mirage, phantasm, phantasma, will-o'-the-wisp. See real/imaginary.

A wavering luminous appearance frequently observed in meadows and marshy places, around which many popular superstitions cluster. Its folknames, Will o' the Wisp and Jack o' Lantern, suggest a country fellow bearing a lantern or straw torch (wisp). Formerly these lights were supposed to haunt desolate bogs and moorlands for the purpose of misleading travelers and drawing them to their death. Another superstition says that they are the spirits of those who have been drowned in the bogs, and yet another says that they are the souls of unbaptized infants. Science now attributes these ignes fatui to gaseous exhalations from the moist ground or, more rarely, to night-flying insects.

[L., foolish fire] deceptive goal or hope; light from marsh gas