Source : Webster's Revised Unabridged Dictionary (1913)

Will-o'-the-wisp \Will"-o'-the-wisp`\, n.
   See {Ignis fatuus}.

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}.

Source : WordNet®

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