What is "will-with-the-wisp"

Ignis fatuus \Ig"nis fat"u*us\; pl. Ignes fatui. [L. ignis fire + fatuus foolish. So called in allusion to its tendency to mislead travelers.]

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.

2. Fig.: A misleading influence; a decoy.

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