Wikipedia
Toparches (, "place-ruler"), anglicized as toparch, is a Greek term for a governor or ruler of a district, which in Byzantine times came to be applied to independent or semi-independent rulers in the periphery of the Byzantine world.
The term appears in Hellenistic times, and remained in use under the Roman Empire in the Greek East, for the governor of a district. Such districts were then called "toparchies" (sing. toparchy, from Greek τοπαρχία, toparchia). In the 6th century, in the Novellae Constitutiones of Emperor Justinian I, the term was used to encompass all local magistrates, both civilian and military.
More often, however, Byzantine writers use the term to refer to local monarchs, especially during the 10th–13th centuries, when, according to the Byzantinist Paul Lemerle, "a toparches is the independent ruler of a foreign territory adjoining the Empire... He is in some manner under the influence of the Empire, as it is supposed that he may rebel against the Byzantines". This usage extended not only to actual breakaway or de facto autonomous Byzantine governors, who appear during the military crises and administrative disintegration of the 11th–12th centuries, but was also applied to independent rulers, usually on the periphery of the Byzantine Empire (e.g. the Emir of Crete, various Turkish lords in Anatolia, or the rulers of Bulgaria or Serbia), of territories which the Byzantines considered rightfully theirs.
In this context, the late 11th-century writer Kekaumenos dedicates a large part of his Strategikon to advising the toparches on his conduct and dealings with the emperor and the other Byzantine governors.