Each of these was subdivided into dioceses, each under a vicarius, and these again into provinciae or eparchies, i.e. Roman provinces (but smaller than before, in many cases resulting from the split of a pre-existing province, and thus more numerous), under governors with different ranks (in many cases praeses provinciae, but also various terms tied into the pre-Dominate vocabulary) reflecting the province's intrinsic and/or strategic importance, for which the generic Latin term rector was used.
Notwithstanding the primacies of the Apostolic Sees of Rome, Alexandria and Antioch, the bishoprics of one civil province were grouped together in church provinces, also called eparchies, under the supervision of the metropolitan, usually the bishop of the provincial capital. The First Council of Nicaea in 325 accepted this arrangement and orders that: "the authority [of appointing bishops] shall belong to the metropolitan in each eparchy" (can. iv), i.e. in each such civil eparchy there shall be a metropolitan bishop who has authority over the others.
Later in Eastern Christendom, after a process of title-inflation, multiplying the numbers of dioceses, metropolitans and (arch)bishops and reducing their territorial size, the use of the word was gradually modified and came to refer to the diocese of a bishop. This usage is prevalent in Eastern and Oriental Orthodoxy and the Eastern Catholic Churches.
The name Eparchy was, however, not commonly used except in Russia, as the usual term for a diocese. The Russian Orthodox Church in the early 20th century counted 86 eparchies, of which three (Kiev, Moscow, and St. Petersburg) were ruled by bishops who always bore the title "Metropolitan", and fourteen others under archbishops. In 1917 an All-Russian Sobor restored the patriarchate and Saint Tikhon was elected the first Patriarch of Moscow since the 17th century.