The Roman Empire was the post-Republican period of the ancient Roman civilization, which was characterized by an autocratic form of government and large territorial holdings in Europe, Asia Minor, and the Mediterranean region. The Roman Empire began in 27 BC, when Octavian took power after the Battle of Actium and was granted the honorific title of Augustus, becoming the first Roman emperor, Emperor Augustus. The 500-year-old Roman Republic, which preceded it, had been weakened by several civil wars. Several events are commonly proposed to mark the transition from Republic to Empire, including Julius Caesar’s appointment as perpetual dictator in 44 BC, the victory of Octavian at the Battle of Actium on September 2, 31 BC), and the Roman Senate’s granting to Octavian the honorific title of Augustus on January 16, 27 BC.
The Latin term Imperium Romanum, Roman Empire, indicates the part of the world under Roman rule. Roman expansion began in the days of the Republic, but reached its zenith under Emperor Trajan. At this territorial peak, the Roman Empire controlled approximately 5,900,000 km² (2,300,000 sq mi) of land surface. Because of the Empire’s vast extent and long endurance, Roman influence upon the language, religion, architecture, philosophy, law, and government of nations around the world lasts to this day.
In 285 AD, Emperor Diocletian divided authority between two emperors, one in the western part of the empire and one in the east, in order to better administer the vast territory. For the next century this division of emperial power continued, with occasional periods in which one emperor assumed complete control. However, after the death of Theodosius in 395, no single emperor would ever again hold genuine supremacy over a united Roman Empire. The Western Roman Empire collapsed in 476 as Romulus Augustus was forced to abdicate by Odoacer. The Eastern Roman or Byzantine Empire endured until 1453 with the capture of Constantinople by the Ottoman Turks led by Mehmed II.
[...] by which a formally republican state could be led by a sole ruler; the result became known as the Roman Empire. The emperorship was never an office like the Roman dictatorship which Caesar and Sulla had held [...]