In folklore, a will-o'-the-wisp, will-o'-wisp, or ignis fatuus, is an atmospheric ghost light seen by travellers at night, especially over bogs, swamps or marshes. The phenomenon is known in much of European folklore by a variety of names, including jack-o'-lantern, friar's lantern, and hinkypunk, and is said to mislead travellers by resembling a flickering lamp or lantern. In literature, will-o'-the-wisp metaphorically refers to a hope or goal that leads one on, but is impossible to reach, or something one finds strange or sinister. Wills-o'-the-wisp appear in folk tales and traditional legends of numerous countries and cultures; notable named examples include St. Louis Light in Saskatchewan, the Spooklight in Southwestern Missouri and Northeastern Oklahoma, the Marfa lights of Texas, the Naga fireballs on the Mekong in Thailand, the Paulding Light in Upper Peninsula of Michigan, and the Hessdalen light in Norway.

Atmospheric ghost lights are lights that appear in the atmosphere without an obvious cause. Examples include the onibi, hitodama and will-o'-wisp. They are often seen in humid climates.