jack-o’-lantern, in meteorology, a mysterious light seen at night flickering over marshes; when approached, it advances, always out of reach. The phenomenon is also known as will-o’-the-wisp and ignis fatuus (Latin: “foolish fire”). In popular legend it is considered ominous and is often purported to be the soul of one who has been rejected by hell carrying its own hell coal on its wanderings. The phenomenon is generally believed to be due to the spontaneous ignition of marsh gas, which consists mostly of methane and which is produced by the decomposition of dead plant matter.