will-o'-the-wisp [wĭl.ə.ðə`wĭsp] noun.

A ball of light, usually over marshes or bogs, that floats hither and thither and which there is no confirmed explanation for.
[Fr. Will, a wicked blacksmith in English folklore who spoiled a second chance given him by St. Peter. He was given a piece of burning coal to warm him as he wandered. He used it to lead travellers astray]
Also called peg-a-lantern, jack-o'-lantern, jack-a-lantern, jack-o'-lanthorn, jack-with-a-lantern, jacky lantern, joan-in-the-wad, kit-o'-the-canstick, kit-with-the-canstick, Billy-wi'-t'-wisp, bob-a-longs, corpse light, elf fire, fairy light, fox fire, hob-and-his-lanthorn, merry dancers, nimble men, friar's lantern, St. Elmo's fire, spunkies, will-o'-the-wikes, pinket, hinky punk, Jenny-with-the-lantern, Jenny-burnt-tail, walking fire, and ignis fatuus