A Jack o'lantern is a pumpkin whose top and stem have been carved off and inner membranes and seeds scooped out to leave a hollow shell. Sections of a side are carved out to make a design, usually a face. It is possible, using thicker and thinner sections cut with differing tools, to create surprisingly detailed and realistic designs. A light source (traditionally a candle) is placed inside the pumpkin and the top is put back into place (often after a "chimney" is carved in the lid in order to allow heat to escape). The light illuminates the design from the inside. Jack o'lanterns are generally made for Halloween, and were originally made from turnips, beets and rutabagas. The practice of carving jack o'lanterns began in Ireland.
The practice of carving Jack o'lanterns goes back to the Irish legend of Jack, a lazy but shrewd farmer who tricked the Devil into a tree, then refused to let the Devil down unless the Devil agreed to never let Jack into Hell. The Devil agreed. When Jack died, he was too sinful to be allowed into Heaven, but the Devil wouldn't let him into Hell. So, Jack carved out one of his turnips, put a candle inside it, and began endlessly wandering the Earth for a resting place. He was known as Jack of the Lantern, or Jack O'Lantern.
For the mushroom Omphalotus olearius, please see Jack O'Lantern (mushroom).