Ignis Fatuus, a pale, greenish or bluish light sometimes seen hovering over swamps and marshes at night. The light is also called will-o'-the-wisp or jack-o'-lantern. Ignis fatuus is a Latin expression meaning foolish fire, so called because of a superstition that the light misleads travelers. The term is also used figuratively in referring to a misleading influence or impractical scheme.

The light was once believed to be caused by luminous insects or phosphorescence of decaying vegetable matter. Scientists today, however, believe that it is due to the spontaneous combustion of marsh gas, which is chiefly methane. This gas is produced by the decomposition of organic substances in swamps and stagnant water. In coal mines the gas is commonly called firedamp. It is the cause of many explosions.