An emperor (from , via ) is the (male) ruler of an empire. Empress, the female equivalent, may indicate an emperor's wife (empress consort), mother/grandmother (empress dowager/grand empress dowager), or a woman who rules in her own right and name (empress regnant or suo jure). Emperors are generally recognised to be of the highest monarchic honour and rank, surpassing kings. In Europe, the title of Emperor has been used since the Middle Ages, considered in those times equal or almost equal in dignity to that of Pope due to the latter's position as visible head of the Church and spiritual leader of the Catholic part of Western Europe. The emperor of Japan is the only currently reigning monarch whose title is translated into English as "Emperor".
Both emperors and kings are monarchs or sovereigns, both emperor and empress are considered monarchical titles. In as much as there is a strict definition of emperor, it is that an emperor has no relations implying the superiority of any other ruler and typically rules over more than one nation. Therefore, a king might be obliged to pay tribute to another ruler, or be restrained in his actions in some unequal fashion, but an emperor should in theory be completely free of such restraints. However, monarchs heading empires have not always used the title in all contexts—the British sovereign did not assume the title Empress of the British Empire even during the incorporation of India, though she was declared Empress of India.
In Western Europe, the title of Emperor was used exclusively by the Holy Roman Emperor, whose imperial authority was derived from the concept of , i.e., they claimed succession to the authority of the Roman emperors, thus linking themselves to Roman institutions and traditions as part of state ideology. Although initially ruling much of Central Europe and northern Italy, by the 19th century, the emperor exercised little power beyond the German-speaking states.
Although technically an elective title, by the late 16th century, the imperial title had in practice come to be inherited by the Habsburg Archdukes of Austria and, following the Thirty Years' War, their control over the states (outside the Habsburg monarchy, i.e. Austria, Bohemia and various territories outside the empire) had become nearly non-existent. However, Napoleon Bonaparte was crowned Emperor of the French in 1804 and was shortly followed by Francis II, Holy Roman Emperor, who declared himself Emperor of Austria in the same year. The position of Holy Roman Emperor nonetheless continued until Francis II abdicated that position in 1806. In Eastern Europe, the monarchs of Russia also used to wield imperial authority as successors to the Eastern Roman Empire. Their status was officially recognised by the Holy Roman Emperor in 1514, although not officially used by the Russian monarchs until 1547. However, the Russian emperors are better known by their Russian-language title of Tsar even after Peter the Great adopted the title of Emperor of All Russia in 1721.
Historians have liberally used "emperor" and "empire" anachronistically and out of its Roman and European context to describe any large state from the past or the present. Some titles are considered equivalent to "emperor" or are translated as "emperor". Examples of that are Roman emperors' titles, King of Kings, Khalifa, Huangdi, Cakravartin, Great Khan, Aztec monarchs' title, Inca monarchs' title, etc. Sometimes this reference has even extended to non-monarchically ruled states and their spheres of influence, such as the Athenian Empire of the late 5th century BC, the Angevin Empire of the Plantagenets and the Soviet and US "empires" of the Cold War era. However, such "empires" did not need to be headed by an "emperor". "Empire" became identified instead with vast territorial holdings rather than the title of its ruler by the mid-18th century.
For purposes of protocol, the size and scope of a kingdom or empire may determine precedence in international diplomatic relations, but currently, precedence among heads of state who are sovereigns—whether they be kings, queens, emperors, empresses, princes, princesses and presidents may be determined by the size and scope or time that each one has been continuously in office. Outside the European context, "emperor" was the translation given to holders of titles who were accorded the same precedence as European emperors in diplomatic terms. In reciprocity, these rulers might accredit equal titles in their native languages to their European peers. Through centuries of international convention, this has become the dominant rule to identifying an emperor in the modern era.
Roman Empire
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 (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 , Augoustos or translated as , 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 (autokratōr, only partly overlapping with the modern understanding of "autocrat") or (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.