ig·nis fat·u·us /ˈɪgnəsˈfæʧəwəs, ˈfæʧwəs/



 Ig·nis fat·u·us pl. Ignes fatui
 1. A phosphorescent light that appears, in the night, over marshy ground, supposed to be occasioned by the decomposition of animal or vegetable substances, or by some inflammable gas; -- popularly called also Will-with-the-wisp, or Will-o'-the-wisp, and Jack-with-a-lantern, or Jack-o'-lantern.  It is thought by some to be caused by phosphine, PH3, a sponaneously combustible gas.
 2. Fig.: A misleading influence; a decoy.
    Scared and guided by the ignis fatuus of popular superstition.   --Jer. Taylor.

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 ignis fatuus
      n 1: a pale light sometimes seen at night over marshy ground
           [syn: friar's lantern, jack-o'-lantern, will-o'-the-wisp]
      2: an illusion that misleads [syn: will-o'-the-wisp]