Roman emperor
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.
Roman emperor
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.