Ignis Fatuus means strictly a fatuous fire it is also called "Jack o' Lantern," "Spunkie," "Walking Fire,"
"Will o' the Wisp," and "Fair Maid of Ireland." Milton calls it Friar's Lanthern, and Sir Walter Scott
Friar Rush with a lantern. Morally speaking, a Utopian scheme, no more reducible to practice than the
meteor so called can be turned to any useful end. (Plural, Ignes fatui.)

A mystery to the ancients, it is now recognized that the tiny flames oocasionally seen above swampy
places are due to spontaneous ignition of methane produced by decomposition of cellulose and other
such things below the surface.

The word fatuus means illusory or unreal.