I found the following definitions for Jack-o-lanterns.
1. A lantern made from a hollowed pumpkin with openings representing eyes, nose, and mouth to make it look like a face—a symbol of Halloween.
2. The light of burning marsh gas.
The symbol of Halloween in North America is the reduction of an old phrase, "jack with a lantern."
In Play: The custom of putting carved vegetables out on Halloween did, however, originate with Irish Catholics. The Irish once placed carved turnips and rutabagas containing candles in their windows to ward off the dead souls they presumed wandered about on the eve of All Saints Day, originally known as All Hallow Even(ing). They switched to pumpkins when they immigrated to America since turnips and rutabagas were more likely to be served for dinner.
(Somehow, I doubt my kids would be satisfied with carving rutabagas and turnips instead of pumpkins.)
Jack-with-a-lantern originally meant "man with a lantern" (jack, as in the phrase, "every man, jack of them"). It referred to a night watchman. Its later structure, jack-o'-lantern, is analogical with that of will-o'-the-wisp, which originally meant only "a man named Will with a wisp (whiskbroom)". Both will-o'-the-wisp and jack-o'-lantern were later used to refer to what the Romans called ignis fatuus "crazy fire", the pale, mysterious fire from gas that sometimes faintly burns over marshy areas. A will-o'-the-wisp was then taken to be a sprite carrying the wisp of a torch across the swamps. A jack-o'-lantern was assumed to be a man with a lantern engaged in the same activity.
And that's the rest of the story.
Everybody have a safe and fun Halloween. We'll be trick-or-treating and then watching scary movies. Oh! And of course OD'ing on chocolate candy.