Lantern tower

In architecture, the lantern tower is a tall construction above the junction of the four arms of a cruciform (cross-shaped) church, with openings through which light from outside can shine down to the crossing. Many lantern towers are usually octagonal, and give an extra dimension to the decorated interior of the dome.

Lantern tower on Tarazona's Cathedral, Aragón, (Spain).

An affiliated term is the Italian tiburio, derived from the Latin-derived word of tiburium, which reflects the often polygonal lantern atop a dome. It is often interspersed with windows both to lighten the load and allow for light to shine.