Omphalion
Omphalion in Greek means "navel (of the earth)"; compare the omphalos of Delphi.
Hagia Sophia
The Omphalion is a group of circular marble slabs fitted into a design on the main floor of the Great Church of Hagia Sophia (Church of the Holy Wisdom) in Constantinople (modern Istanbul).
The Omphalion is located in front of the Ottoman muezzin lodge. In the centre (the larger slab) is the location where the coronation of every Byzantine Basileus (Greek: Βασιλεύς) or Emperor, took place.
This is also the spot where the Emperor would sit and participate during church ceremonies.
Other Omphalia
The word omphalion is also used to describe other sacred circular marking slabs that were usually located in important or especially sacred churches throughout the Byzantine Empire.
The marble omphalion from the floor of the Church of Hagia Sophia in Trabzon (north coast of modern Turkey) was brought to Thessalonica by Greek refugees in 1924.