Cenobiarch
The top priest or Superior Iam in the Omnian Church.
Annotations
The office of the Cenobiarch bears a great resemblance to that of the Roundworld Pope, or the Islamic Caliph.
The word 'Cenobiarch' seems to come from the word 'cenobite', meaning "member of a religious order following a communal way of life". (From APF). The '-arch' indicates superiority and leadership, thus somewhat belying the communal aspect (in the fine tradition of 'some are more equal than others').
The title also has resonance with Theodosius the Cenobiarch (c. 423-529): a monk, abbot, and saint who was a staunch opponent of Monophysitism - this itself has more than a passing synonimity with Discworld Omnianism. (Source: Wikipedia)