What is the main church of a Byzantine monastery called?

The main church of a Byzantine monastery is called the katholikon.

The Greek word katholikon means the principal or chief church of a monastery. It served the community’s central liturgical life, while smaller chapels could be dedicated to particular saints or functions.

A katholikon commonly stood near other monastic buildings, including monks’ cells, refectories, storage rooms, and reception areas. In Byzantine monasteries, its architecture often used the cross-in-square plan, a central dome, a sanctuary apse, and a narthex.

The word is not limited to one architectural shape. A katholikon may have a variety of plans depending on its date, region, and monastic foundation. It is also different from the word cathedral, which identifies the principal church of a bishop’s seat rather than a monastery.