Ignis fatuus

Ignis fatuus  (ig ´niss fat ´u-us ).—A deceiving fire: a Will-o'-the-wisp; an inflammable gas frequently seen over marshes, which leads the traveler who pursues it into the bog.
Ignis fatuus  (ĭg ´-nĭs făt ´ûŭs ).—A phosphorescent light that appears, in the night, over marshy grounds, 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.