The traditional role of a bishop is as pastor of a diocese (also called a bishopric, synod, eparchy or see), and so to serve as a "diocesan bishop," or "eparch" as it is called in many Eastern Christian churches. Dioceses vary considerably in size, geographically and population-wise. Some dioceses around the Mediterranean Sea which were Christianised early are rather compact, whereas dioceses in areas of rapid modern growth in Christian commitment—as in some parts of Sub-Saharan Africa, South America and the Far East—are much larger and more populous.