In Celtic folklore, what flickering light is said to lead travelers astray?

In Celtic folklore, the flickering light said to lead travelers astray is a will-o'-the-wisp.

A will-o'-the-wisp is a mysterious ghost light traditionally seen at night over bogs, marshes, and other wet ground. Folklore often describes it as a distant lamp that dances, recedes, or lures travelers away from safe paths. In Scottish tradition, a related figure called the spunkie could mislead people on roads or even cause sailors to mistake it for a harbor light.

The name is one of many regional labels for the same tradition. “Ignis fatuus” is the Latin term, while Ireland has names including tine ghealáin, meaning “flashing fire.” Jack-o'-lantern can also refer to this phenomenon in older British and Irish folklore, although the modern Halloween lantern is a different cultural object.

Natural explanations have included gases released by decaying organic matter, especially methane and phosphorus compounds, as well as mistaken distant lights and atmospheric effects. The scientific explanation does not erase the folklore: the will-o'-the-wisp remains a classic symbol of deceptive guidance and perilous travel.