A jack-o'-lantern is a pumpkin with a face cut into it and a light inside — usually a candle, now often a small electric one — so the eyes and mouth glow through the holes. It is the Halloween pumpkin you see on steps and in windows. Though actually, the name is older than the pumpkin. It first named a night watchman who carried a lantern (the lantern itself was once often a hollow turnip). Later, in English folklore, jack-o'-lantern was also a name for those strange lights over marshes, the kind of ghost-light also called a will-o'-the-wisp. The carved-pumpkin sense is the one learners meet now, from the 1800s on. Jack is again that stand-in man’s name; o' is a short of; lantern is the light. Informal, seasonal, friendly. If you only mean a lamp you carry, say lantern; a jack-o'-lantern is specifically that carved, glowing face.