Classical Antiquity
When Republican Rome turned into a de facto monarchy in the second half of the 1st century BC, at first there was no name for the title of the new type of monarch. Ancient Romans abhorred the name Rex ("king"), and it was critical to the political order to maintain the forms and pretences of republican rule. Julius Caesar had been Dictator, an acknowledged and traditional office in Republican Rome. Caesar was not the first to hold it, but following his assassination the term was abhorred in Rome.
Augustus, considered the first Roman emperor, established his hegemony by collecting on himself offices, titles, and honours of Republican Rome that had traditionally been distributed to different people who held these roles for a specific time period, often with a colleague. In the process, Augustus concentrated what had been distributed power in one man, all at once, and in practice without a time limit (the powers with nominal time limits were renewed when they lapsed). One of these offices was princeps senatus, ("first man of the Senate") and became changed into Augustus' chief honorific, princeps civitatis ("first citizen") from which the modern English word and title prince is descended as for the term premier. The first period of the Roman Empire, from 27 BC to AD 284, is called the principate for this reason. However, it was the informal descriptive of Imperator ("commander") that became the title increasingly favoured by his successors. Previously bestowed on high officials and military commanders who had imperium, Augustus reserved it exclusively to himself as the ultimate holder of all imperium. (Imperium is Latin for the authority to command, one of a various types of authority delineated in Roman political thought.)
Beginning with Augustus, Imperator appeared in the title of all Roman monarchs through the extinction of the Empire in 1453. After the reign of Augustus' immediate successor Tiberius, being proclaimed imperator was transformed into the act of accession to the head of state. Other honorifics used by the Roman emperors have also come to be synonyms for Emperor:
Caesar Latin: [ˈkae̯sar] (as, for example, in Suetonius' Twelve Caesars). This tradition continued in many languages: in German it became "Kaiser"; in certain Slavic languages it became "Tsar"; in Hungarian it became "Császár", and several more variants. The name derived from Julius Caesar's cognomen "Caesar": this cognomen was adopted by all Roman emperors, exclusively by the ruling monarch after the Julio-Claudian dynasty had died out. In this tradition Julius Caesar is sometimes described as the first Caesar/emperor (following Suetonius). This is one of the most enduring titles: Caesar and its transliterations appeared in every year from the time of Caesar Augustus to the modern era.
Augustus was the honorific first bestowed on Emperor Augustus: on his death it became an official title of his successor and all Roman emperors after him added it to their name. Although it had a high symbolic value, something like "elevated" or "sublime", it was generally not used to indicate the office of Emperor itself. Exceptions include the title of the Augustan History, a semi-historical collection of emperors' biographies of the 2nd and 3rd century. This title also proved very enduring: after the fall of the Roman Empire, the title would be incorporated into the style of the Holy Roman Emperor, a precedent set by Charlemagne, and its Greek translation Sebastos continued to be used in the Byzantine Empire until the Fall of Constantinople in 1453, although it gradually lost its imperial exclusivity. Augustus had (by his last will) granted the feminine form of this honorific (Augusta) to his wife. Since there was no "title" of Empress(-consort) whatsoever, women of the reigning dynasty sought to be granted this honorific, as the highest attainable goal. Few were however granted the title, and it was certainly not a rule that all wives of reigning emperors would receive it.
Imperator (as, for example, in Pliny the Elder's Naturalis Historia). In the Roman Republic Imperator meant "(military) commander". In the late Republic, as in the early years of the new monarchy, Imperator was a title granted to Roman generals by their troops and the Roman Senate after a great victory, roughly comparable to field marshal (head or commander of the entire army). For example, in AD 15 Germanicus was proclaimed Imperator during the reign of his adoptive father Tiberius. Soon thereafter "Imperator" became however a title reserved exclusively for the ruling monarch. This led to "Emperor" in English and, among other examples, "Empereur" in French and "Mbreti" in Albanian. The Latin feminine form Imperatrix only developed after "Imperator" had taken on the connotation of "Emperor".
Autokrator (Αὐτοκράτωρ) or Basileus (βασιλεύς): although the Greeks used equivalents of "Caesar" (Καῖσαρ, Kaisar) and "Augustus" (in two forms: transliterated as Αὔγουστος, Augoustos or translated as Σεβαστός, Sebastos) these were rather used as part of the name of the emperor than as an indication of the office. Instead of developing a new name for the new type of monarchy, they used αὐτοκράτωρ (autokratōr, only partly overlapping with the modern understanding of "autocrat") or βασιλεύς (basileus, until then the usual name for "sovereign"). Autokratōr was essentially used as a translation of the Latin Imperator in Greek-speaking part of the Roman Empire, but also here there is only partial overlap between the meaning of the original Greek and Latin concepts. For the Greeks Autokratōr was not a military title, and was closer to the Latin dictator concept ("the one with unlimited power"), before it came to mean Emperor. Basileus appears not to have been used exclusively in the meaning of "emperor" (and specifically, the Roman/Byzantine emperor) before the 7th century, although it was a standard informal designation of the emperor in the Greek-speaking East. The title was later applied by the rulers of various Eastern Orthodox countries claiming to be the successors of Rome/Byzantium, such as Georgia, Bulgaria, Serbia, Russia.
After the turbulent Year of the Four Emperors in 69, the Flavian dynasty reigned for three decades. The succeeding Nervan-Antonian dynasty, ruling for most of the 2nd century, stabilised the empire. This epoch became known as the era of the Five Good Emperors, and was followed by the short-lived Severan dynasty.
During the Crisis of the 3rd century, barracks emperors succeeded one another at short intervals. Three short lived secessionist attempts had their own emperors: the Gallic Empire, the Britannic Empire, and the Palmyrene Empire though the latter used rex more regularly.
The Principate (27 BC – 284 AD) period was succeeded by what is known as the Dominate (284 AD – 527 AD), during which Emperor Diocletian tried to put the empire on a more formal footing. Diocletian sought to address the challenges of the Empire's now vast geography and the instability caused by the informality of succession by the creation of co-emperors and junior emperors. At one point, there were as many as five sharers of the imperium (see: Tetrarchy). In 325 AD Constantine I defeated his rivals and restored single emperor rule, but following his death the empire was divided among his sons. For a time the concept was of one empire ruled by multiple emperors with varying territory under their control, however following the death of Theodosius I the rule was divided between his two sons and increasingly became separate entities. The areas administered from Rome are referred to by historians the Western Roman Empire and those under the immediate authority of Constantinople called the Eastern Roman Empire or (after the Battle of Yarmouk in 636 AD) the Later Roman or Byzantine Empire. The subdivisions and co-emperor system were formally abolished by Emperor Zeno in 480 AD following the death of Julius Nepos last Western Emperor and the ascension of Odoacer as the de facto King of Italy in 476 AD.
