A dungeon is an underground prison or cell, usually in a castle or fortress. In medieval times, dungeons were dark, cold chambers carved from stone where prisoners were locked away, sometimes for years. The word comes from Old French donjon, which originally referred to the lord's tower or keep, the strongest, most secure part of a castle. Over time, it came to mean specifically the gloomy cells hidden beneath that tower.

Real medieval dungeons were grim places. They had thick stone walls, tiny windows (if any), and heavy iron doors. Some were so deep underground that prisoners never saw daylight. Guards might lower food through a hole in the ceiling. The dampness, darkness, and isolation made dungeons one of the most feared punishments of the Middle Ages.

Today, the word appears most often in fantasy games and stories. In Dungeons & Dragons and similar games, a dungeon is any dangerous underground maze filled with monsters, traps, and treasure. Players explore these imaginary dungeons room by room, battling creatures and solving puzzles. The dungeon in these games keeps the medieval feeling of danger and mystery but adds adventure and excitement.

You might also hear someone jokingly call a dark basement or cramped room a dungeon: “My locker is so small and dark, it's like a dungeon!”