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.]

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

Note: click on a word meaning below to see its connections and related words.

The noun has 2 meanings:

Meaning #1: a pale light sometimes seen at night over marshy ground
  Synonyms: friar's lantern, jack-o'-lantern, will-o'-the-wisp

Meaning #2: an illusion that misleads
  Synonym: will-o'-the-wisp

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