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.
Emperor
From the time of Otto the Great onward, much of the former Carolingian kingdom of Eastern Francia became the Holy Roman Empire. The prince-electors elected one of their peers as King of the Romans and King of Italy before being crowned by the Pope. The Emperor could also pursue the election of his heir (usually a son) as King, who would then succeed him after his death. This junior King then bore the title of Roman King (King of the Romans). Although technically already ruling, after the election he would be crowned as emperor by the Pope. The last emperor to be crowned by the pope was Charles V; all emperors after him were technically "emperors-elect", but were universally referred to as "Emperor".