ig·nis fat·u·us 
(ĭgnĭs făch-əs)
(ĭgnĭs făch-əs)n. pl. ig·nes fat·u·i (ĭgnēz făch-ī′)
1. A phosphorescent light that hovers or flits over swampy ground at night, possibly caused by spontaneous combustion of gases emitted by rotting organic matter. Also called friar's lantern, jack-o'-lantern, will-o'-the-wisp, wisp.
2. Something that misleads or deludes; an illusion.
[Medieval Latin : Latin ignis, fire + Latin fatuus, foolish.]