The dome over the center of a Byzantine cross-in-square church is its central dome.
The cross-in-square plan became one of the defining church forms of middle Byzantine architecture. Four vaulted arms extend from a central bay, and the central dome rises above that bay to emphasize the intersection of the cross.
The dome was normally supported by four main piers or columns, with transitional structures helping carry its circular base. Smaller domes or vaults could cover the corner bays, while the eastern arm ended in the sanctuary apse. This arrangement created a compact church with a strong vertical focus.
The phrase “central dome” is descriptive rather than a uniquely Byzantine technical noun. It should not be confused with a pendentive, which is a structural curved surface used to transition from a square space to a circular dome base.