The Roman emperor was the ruler of the Roman Empire during the imperial period (starting in 27 BC). The emperors used a variety of different titles throughout history. Often when a given Roman is described as becoming "emperor" in English, it reflects his taking of the title Augustus or Caesar. Another title often used was imperator, originally a military honorific. Early Emperors also used the title princeps (first citizen). Emperors frequently amassed republican titles, notably princeps senatus, consul and pontifex maximus.
The legitimacy of an emperor's rule depended on his control of the army and recognition by the Senate; an emperor would normally be proclaimed by his troops, or invested with imperial titles by the Senate, or both. The first emperors reigned alone; later emperors would sometimes rule with co-emperors and divide administration of the empire between them.
The Romans considered the office of emperor to be distinct from that of a king. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, for the first three hundred years of Roman emperors, from Augustus until Diocletian, efforts were made to portray the emperors as leaders of a republic.
From Diocletian, whose tetrarchic reforms also divided the position into one emperor in the West and one in the East, until the end of the Empire, emperors ruled in an openly monarchic style and did not preserve the nominal principle of a republic, but the contrast with "kings" was maintained: although the imperial succession was generally hereditary, it was only hereditary if there was a suitable candidate acceptable to the army and the bureaucracy, so the principle of automatic inheritance was not adopted. Elements of the Republican institutional framework (senate, consuls, and magistrates) were preserved until the very end of the Western Empire.
The Western Roman Empire collapsed in the late 5th century. Romulus Augustulus is often considered to be the last emperor of the West after his forced abdication in 476, although Julius Nepos maintained a claim recognized by the Eastern Empire to the title until his death in 480. Following Nepos' death, the Eastern Emperor Zeno abolished the division of the position and proclaimed himself as the sole Emperor of a reunited Roman Empire. The Eastern imperial lineage continued to rule from Constantinople ("New Rome"); they continued to style themselves as Emperor of the Romans (later βασιλεύς Ῥωμαίων in Greek), but are often referred to in modern scholarship as Byzantine emperors. Constantine XI Palaiologos was the last Roman emperor in Constantinople, dying in the Fall of Constantinople to the Ottomans in 1453.
The "Byzantine" emperors from Heraclius in 629 and onwards adopted the title of basileus (βασιλεύς), which had originally meant king in Greek but became a title reserved solely for the Roman emperor and the ruler of the Sasanian Empire. Other kings were then referred to as rēgas.
In addition to their pontifical office, some emperors were given divine status after death. With the eventual hegemony of Christianity, the emperor came to be seen as God's chosen ruler, as well as a special protector and leader of the Christian Church on Earth, although in practice an emperor's authority on Church matters was subject to challenge.
Due to the cultural rupture of the Turkish conquest, most western historians treat Constantine XI as the last meaningful claimant to the title Roman Emperor. From 1453, one of the titles used by the Ottoman Sultans was "Caesar of Rome" (Turkish: Kayser-i Rum), part of their titles until the Ottoman Empire ended in 1922. A Byzantine group of claimant Roman emperors existed in the Empire of Trebizond until its conquest by the Ottomans in 1461, though they had used a modified title since 1282.
Eastern emperors in Constantinople had been recognized and accepted as Roman emperors both in the East, which they ruled, and by the Papacy and Germanic kingdoms of the West until the deposition of Constantine VI and accession of Irene of Athens as Empress regnant in 797. Objecting to a woman ruling the Roman Empire in her own right and issues with the eastern clergy, the Papacy would then create a rival lineage of Roman emperors in western Europe, the Holy Roman Emperors, which ruled the Holy Roman Empire for most of the period between 800 and 1806. These Emperors were never recognized as Roman emperors by the court in Constantinople.
At the end of the Roman Republic no new, and certainly no single, title indicated the individual who held supreme power. Insofar as emperor could be seen as the English translation of imperator, then Julius Caesar had been an emperor, like several Roman generals before him. Instead, by the end of the civil wars in which Julius Caesar had led his armies, it became clear that there was certainly no consensus to return to the old-style monarchy, but that the period when several officials, bestowed with equal power by the senate, would fight one another had come to an end.
Julius Caesar, and then Augustus after him, accumulated offices and titles of the highest importance in the Republic, making the power attached to those offices permanent, and preventing anyone with similar aspirations from accumulating or maintaining power for themselves. However, Julius Caesar, unlike those after him, did so without the Senate's vote and approval.
Julius Caesar held the Republican offices of consul four times and dictator five times, was appointed dictator in perpetuity (dictator perpetuo) in 45 BC and had been "pontifex maximus" for a long period. He gained these positions by senatorial consent. By the time of his assassination, he was the most powerful man in the Roman world.
In his will, Caesar appointed his adopted son Octavian as his heir. On Caesar's death, Octavian inherited his adoptive father's property and lineage, the loyalty of most of his allies and – again through a formal process of senatorial consent – an increasing number of the titles and offices that had accrued to Caesar. A decade after Caesar's death, Octavian's victory over his erstwhile ally Mark Antony at Actium put an end to any effective opposition and confirmed Octavian's supremacy.
In 27 BC, Octavian appeared before the Senate and offered to retire from active politics and government; the Senate not only requested he remain, but increased his powers and made them lifelong, awarding him the title of Augustus (the elevated or divine one, somewhat less than a god but approaching divinity). Augustus stayed in office until his death; the sheer breadth of his superior powers as princeps and permanent imperator of Rome's armies guaranteed the peaceful continuation of what nominally remained a republic. His "restoration" of powers to the Senate and the people of Rome was a demonstration of his auctoritas and pious respect for tradition.
Some later historians such as Tacitus would say that even at Augustus' death, the true restoration of the Republic might have been possible. Instead, Augustus actively prepared his adopted son Tiberius to be his successor and pleaded his case to the Senate for inheritance on merit. The Senate disputed the issue but eventually confirmed Tiberius as princeps. Once in power, Tiberius took considerable pains to observe the forms and day-to-day substance of republican government.