A governor is an administrative leader and head of a polity or political region, ranking under the head of state and in some cases, such as governors-general, as the head of a state's official representative. Depending on the type of political region or polity, a governor may be either appointed or elected, and the governor's powers can vary significantly, depending on the public laws in place locally. The adjective pertaining to a governor is gubernatorial, from the Latin root gubernare.
Ancient empires
Pre-Roman empires
Though the legal and administrative framework of provinces, each administered by a governor, was created by the Romans, the term governor has been a convenient term for historians to describe similar systems in antiquity. Indeed, many regions of the pre-Roman antiquity were ultimately replaced by Roman 'standardized' provincial governments after their conquest by Rome. Plato used the metaphor of turning the Ship of State with a rudder; the Latin word for rudder is gubernaculum.
Egypt
In Pharaonic times, the governors of each of the various provinces in the kingdoms of Upper and Lower Egypt (called "nomes" by the Greeks, and whose names often alluded to local patterns of religious worship) are usually known by the Greek word.
Pre- and Hellenistic satraps
Media and Achaemenid Persia introduced the satrapy, probably inspired by the Assyrian / Babylonian examples Alexander the Great and equally Hellenistic diadoch kingdoms, mainly Seleucids (greater Syria) and Lagids ('Ptolemies' in Hellenistic Egypt) in later Persia, again under Iranian dynasties: Parthia the Sassanid dynasty dispensed with the office after Shapur I (who had still 7 of them), replacing them with petty vassal rulers, known as shahdars
Ancient Rome
From the creation of the earliest Roman subject provinces, a governor was appointed each year to administer each of them. The core function of a Roman governor was as a magistrate or judge, and the management of taxation and the public spending in their area.
Under the Republic and the early Empire, however, a governor also commanded military forces in his province. Republican governors were all men who had served in senior magistracies (the consulate or praetorship) in Rome in the previous year, and carried related titles as governor (proconsul or propraetor). The first emperor, Octavianus Augustus (who acquired or settled a number of new territories; officially his style was republican: Princeps civitatis), divided the provinces into two categories; the traditionally prestigious governorships remained as before (in what have become known as "senatorial" provinces), while in a range of others, he retained the formal governorship himself, delegating the actual task of administration to appointees (usually with the title legatus Augusti). The legatus sometimes would appoint a prefect (later procurator), usually a man of equestrian rank, to act as his deputy in a subregion of the larger province: the infamous character of Pontius Pilate in the Christian Gospels was a governor of this sort.
A special case was Egypt, a rich 'private' domain and vital granary, where the emperor almost inherited the theocratic status of a pharaoh. The emperor was represented there by a governor sui generis styled praefectus augustalis, a title evoking the religious cult of the emperor.
Emperors Diocletian (see Tetrarchy) and Constantine in the third and fourth centuries AD carried out a root and branch reorganisation of the administration with two main features:
Provinces were divided up and became much more numerous (Italy itself, before the 'colonizing homeland', was brought into the system for the first time); they were then grouped into dioceses, and the dioceses in turn into four praetorian prefectures (originally each under a residing co-emperor); Military responsibilities were removed from governors and given to new officials called comes rei militaris (the comital title was also granted to many court and civilian administrative positions) or dux, later also magister militum.
The prestigious governorships of Africa and Asia remained with the title proconsul, and the special right to refer matters directly to the emperor; the praefectus augustalis in Alexandria and the comes Orientis in Antioch also retained special titles. Otherwise, the governors of provinces had various titles, some known as consularis, some as corrector, while others as praeses. Apart from Egypt and the East (Oriens – viz greater Syria), each diocese was directed by a governor known as a vicarius. The prefectures were directed by praefecti praetorio (greatly transformed in their functions from their role in the early Empire).