From ~500-600 AD, Byzantine architecture distinctly combined the basilica with a symmetrical central floor plan. Many religious structures built during this time resulted in the characteristic Byzantine Greek-Cross-Plan, with a square central mass, four arms of equal length, and a domed roof. To allow such a dome to rest above a square base, either of two devices was used: the squinch (an arch in each of the corners of a square base that transforms it into an octagon) or the pendentive. Byzantine structures featured soaring spaces and sumptuous decoration: marble columns and inlay, mosaics on the vaults, inlaid-stone pavements, and sometimes gold coffered ceilings like the one in the Hagia Sophia. This era of architectural history was transitional and transformational.
Example: Hagia Sophia, Istanbul (c. 537 AD)
First 10 image search results of byzantine architecture