A cupola is a rounded structure on top of a building's roof. Most cupolas have the shape of a dome, or even take the form of a small dome on top of a larger dome.
Although the majority of cupolas are dome-shaped, the small, windowed projection on top of a train caboose is also called a cupola. A raised gun turret on a tank is yet another type of cupola, as is a furnace used for refining metal. The word cupola is Italian, from the Late Latin root cupula, "a little tub," a diminutive form of cupa, "cask or barrel."