Source : Webster's Revised Unabridged Dictionary (1913)

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}.