Eparchy is an anglicizedGreek word (ἐπαρχία), authentically Latinized as eparchia, which can be loosely translated as the rule or jurisdiction over something, such as a province, prefecture, or territory. It has specific meanings both in politics, history and in the hierarchy of the Eastern Christian churches.
Originally eparchy (ἐπαρχίᾱ, eparchia) was the Greek equivalent of the Latin term provincia, one of the districts of the Roman Empire. As such it was used, chiefly in the eastern parts of the Empire, to designate the Roman provinces. The term eparch (Greek: ἔπαρχος, eparchos) however, designating an eparchy's governor, was most usually used to refer to the praetorian prefects (singular in Greek: ἔπαρχος τοῦ πραιτωρίου, "eparch of the praetorium") in charge of the Empire's praetorian prefectures, and to the Eparch of Constantinople, the city's urban prefect.
The Dominate-period administrative system was retained In the Byzantine period of the Empire until the 7th century. As Greek became the Empire's main administrative language, replacing Latin, in the latter 6th century even the provinces of the Exarchate of Ravenna, in reconquered Italy, were termed eparchiae in Greek as well as in Latin.
In the latter half of the 7th century, the old provincial administration was replaced by the thematic system. Even after that however, the term eparchos remained in use until the 840s for the senior administrative official of each thema, under the governing strategos. Thereafter, eparchs are evident in some cases as city governors, but the most important by far amongst them was the Eparch of Constantinople, whose office had wide-ranging powers and functioned continuously until the 13th century.
The term eparchia was revived as one of the administrative sub-provincial units of post-Ottoman independent Greece, the country being divided into nomoi ("Prefectures"), of which in turn some were subdivided into eparchies. From 1887, the eparchies were abolished as actual administrative units, but were retained for some state services, especially finance services and education, as well as for electoral purposes. Before the Second World War, there were 139 eparchies, and after the war, with the addition of the Dodecanese Islands, their number grew to 147. The provinces were abolished in the mainland (but retained for the islands), in the wide-ranging administrative reform implemented in 1997 (the "Kapodistrias Project") and replaced by enlarged municipalities (demoi).
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 as the usual term for a diocese except in Russia. 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.
Sources and references
Look up eparchy in Wiktionary, the free dictionary.