In folklore, a will-o'-the-wisp, will-o'-wisp, or ignis fatuus (Latin for 'foolish flame'; pl. ignes fatui), is an atmospheric ghost light seen by travellers at night, especially over bogs, swamps or marshes.
The phenomenon is known in the United Kingdom by a variety of names, including jack-o'-lantern, friar's lantern, and hinkypunk, and is said to mislead and/or guide travellers by resembling a flickering lamp or lantern. Equivalents of the will-o'-the-wisps appear in European folklore by various names, e.g., ignis fatuus in Latin, feu follet in French, Irrlicht or Irrwisch in Germany, Hessdalen light in Norway. Equivalents occur in traditions of cultures worldwide (cf. § Global terms); e.g., the Naga fireballs on the Mekong in Thailand. In North America the phenomenon is known as the Paulding Light in Upper Peninsula of Michigan, the Spooklight in Southwestern Missouri and Northeastern Oklahoma, and St. Louis Light in Saskatchewan. In Arab folklore it is known as Abu Fanoos.
In folklore, will-o'-the-wisps are typically attributed as ghosts, fairies or elemental spirits meant to reveal a path or direction. These wisps are portrayed as dancing or flowing in a static form, until noticed or followed, in which case they visually fade or disappear. Modern science explains the light aspect as natural phenomena such as bioluminescence or chemiluminescence, caused by the oxidation of phosphine (PH3), diphosphane (P2H4) and methane (CH4), produced by organic decay.
