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. Scared and guided by the ignis fatuus of popular superstition. Jer. Taylor.
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