What is "tholobate"

A tholobate or drum, in architecture, is the upright part of a building on which a dome is raised. It is generally in the shape of a cylinder or a polygonal prism.

In the earlier Byzantine churches, the dome rested direct on the pendentives and the windows were pierced in the dome itself; in later examples, between the pendentive and the dome an intervening circular wall was built, in which the windows were pierced, and this is the type which was universally employed by the architects of the Renaissance, of whose works the best-known example is St. Peter's Basilica at Rome. Other examples of churches of this type are St Paul's Cathedral in London, and the churches of the Les Invalides, the Val-de-Grâce, and the Sorbonne in Paris.