the leading Byzantine government official from the 6th to the 11th century, entrusted with the authority to maintain public order and safety in Constantinople (modern Istanbul), the Byzantine capital. Called the “father of the city,” he ranked just beneath the emperor in importance.

His authority included the direction of the lawcourts, provisioning of the city, and the conduct of trade and industry. His jurisdiction over the corporations and guilds (collegia) of craftsmen and traders was delineated in the Book of the Eparch, probably written in the 9th–10th century. His primary economic concern focussed on guilds such as those of cattle traders, butchers, fishmongers, bakers, and innkeepers, which held monopolies on supplying provisions for the capital. An entire bureau of the government, the secretum, acted to carry out his orders.

In 1028 the eparch Romanus Argyrus married the daughter of the dying emperor Constantine VIII (reigned 1025–28) and was later proclaimed emperor as Romanus III Argyrus (reigned 1028–34). In the 12th century the eparch's most important functions passed to other officials, and under the Palaeologian dynasty (1261–1453) the name survived only as a court title.