thumb|upright=1.35|The Will o' the Wisp and the Snake by [[Hermann Hendrich (1854–1931)]]
In folklore, a will-o'-the-wisp, or will-o'-wisp (, "foolish flame"), 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,