The Roman Empire (Latin: Imperium Romanum [ɪmˈpɛri.ũː roːˈmaːnũː]; Koinē Greek: Βασιλεία τῶν Ῥωμαίων, romanized: Basileía tōn Rhōmaíōn) was the post-Republican period of ancient Rome. As a polity it included large territorial holdings around the Mediterranean Sea in Europe, Northern Africa, and Western Asia ruled by emperors. From the accession of Caesar Augustus to the military anarchy of the third century, it was a principate with Italy as metropole of the provinces and the city of Rome as sole capital (27 BC – 286 AD). After the military crisis, the empire was ruled by multiple emperors who shared rule over the Western Roman Empire (based in Milan and later in Ravenna) and over the Eastern Roman Empire (also known as the Byzantine Empire; centred on Nicomedia and Antioch, later based in Constantinople). Rome remained the nominal capital of both parts until 476 AD, when the imperial insignia were sent to Constantinople, following the capture of Ravenna by the barbarians of Odoacer and the subsequent deposition of Romulus Augustulus. The fall of the Western Roman Empire to Germanic kings, along with the hellenization of the Eastern Roman Empire into the Byzantine Empire, conventionally marks the end of Ancient Rome and the beginning of the Middle Ages.
The predecessor state of the Roman Empire, the Roman Republic (which had replaced Rome's monarchy in the 6th century BC) became severely destabilized in a series of civil wars and political conflicts. In the mid-1st century BC Julius Caesar was appointed as perpetual dictator and then assassinated in 44 BC. Civil wars and proscriptions continued, culminating in the victory of Octavian, Caesar's adopted son, over Mark Antony and Cleopatra at the Battle of Actium in 31 BC. The following year Octavian conquered Ptolemaic Egypt, ending the Hellenistic period that had begun with the conquests of Alexander the Great of Macedon in the 4th century BC. Octavian's power then became unassailable, and in 27 BC the Roman Senate formally granted him overarching power and the new title Augustus, effectively making him the first Roman emperor.
The first two centuries of the Empire saw a period of unprecedented stability and prosperity known as the Pax Romana ("Roman Peace"). Rome reached its greatest territorial expanse during the reign of Trajan (98–117 AD). A period of increasing trouble and decline began with the reign of Commodus (177–192). In the 3rd century the Empire underwent a crisis that threatened its existence, as the Gallic Empire and Palmyrene Empire broke away from the Roman state, and a series of short-lived emperors, often from the legions, led the empire. The empire was reunified under Aurelian (r. 270–275). In an effort to stabilize it, Diocletian set up two different imperial courts in the Greek East and Latin West in 286. Christians rose to positions of power in the fourth century following the Edict of Milan of 313. Shortly after, the Migration Period, involving large invasions by Germanic peoples and by the Huns of Attila, led to the decline of the Western Roman Empire. With the fall of Ravenna to the Germanic Herulians and the deposition of Romulus Augustulus in 476 AD by Odoacer, the Western Roman Empire finally collapsed; the Eastern Roman Emperor Zeno formally abolished it in 480 AD. Nonetheless, some states in the territories of the former Western Roman Empire would later claim to have inherited the supreme power of the emperors of Rome, most notably the Holy Roman Empire. The Eastern Roman Empire survived for another millennium, until Constantinople fell to the Ottoman Turks of Sultan Mehmed II in 1453.
Due to the Roman Empire's vast extent and long endurance, the institutions and culture of Rome had a profound and lasting influence on the development of language, religion, art, architecture, philosophy, law, and forms of government in the territory it governed, and far beyond. The Latin language of the Romans evolved into the Romance languages of the medieval and modern world, while Medieval Greek became the language of the Eastern Roman Empire. The Empire's adoption of Christianity led to the formation of medieval Christendom. Greek and Roman art had a profound impact on the Italian Renaissance. Rome's architectural tradition served as the basis for Romanesque, Renaissance and Neoclassical architecture, and also had a strong influence on Islamic architecture. The corpus of Roman law has its descendants in many legal systems of the world today, such as the Napoleonic Code, while Rome's republican institutions have left an enduring legacy, influencing the Italian city-state republics of the medieval period, as well as the early United States and other modern democratic republics.