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
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.