lantern

In architecture, an ornamental turret erected on the roof or dome of a building to provide light (and also sometimes providing ventilation). A lantern may be square, circular, or polygonal in plan; and its sides may be glazed or open. There are important medieval examples in the cathedrals of York, Ely, Coutances, and at the church of St Ouen at Rouen. Renaissance examples include those on the domes of Florence Cathedral, St Peter's, Rome, and St Paul's, London.