Noun
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.
Fig.: A misleading influence; a decoy.
Source: Webster's dictionaryReason, an Ignis fatuus of the Mind, Which leaves the light of Nature, Sense, behind. John Wilmot, 2nd Earl of Rochester
The Ignis Fatuus is a vapor shining without heat. Isaac Newton
The abdication of Belief Makes the Behavior small Better an ignis fatuus Than no illume at all. Emily Dickinson