(idiomatic, English folklore) Will o' the wisp, a strange light that attracts travellers.
(US, tradition) A vegetable, usually a pumpkin, but alternatively a turnip, carved into the form of a face and lighted within by a candle. Associated chiefly with the holiday Halloween.
Other Word Forms
Origin of jack-o-lantern
From earlier Jack-with-a-lanternman with a lantern, will-o'-the-wisp
American Heritage Dictionary of the English Language, 5th Edition
Jack (“male given name”) + o' + lantern. Attested as a term for ignis fatuus in English folklore in the 1660s, mostly in East Anglia but also in southwestern England. The sense of carved pumpkin in American English is attested in 1834.