The Roman Empire is the phase of the ancient Roman civilization characterized by an autocratic form of government. The Roman Empire succeeded the 500-year-old Roman Republic (510 BC – 1st century BC), which had been weakened by the conflict between Gaius Marius and Sulla and the civil war of Julius Caesar against Pompey the Great. Several dates are commonly proposed to mark the transition from Republic to Empire, including the date of Julius Caesar's appointment as perpetual dictator (44 BC), the victory of Caesar's heir Octavian at the Battle of Actium (September 2 31 BC), and the Roman Senate's granting to Octavian the honorific Augustus. (January 16 27 BC).

The Latin term Imperium Romanum (Roman Empire), probably the best-known Latin expression where the word imperium denotes a territory, indicates the part of the world under Roman rule. From the time of Augustus to the Fall of the Western Empire, Rome's dominion covered all of the following: England and Wales; most of Europe (west of the Rhine and south of the Alps); coastal northern Africa, together with the adjacent province of Egypt; the Balkans, the Black Sea, and Asia Minor; and also much of the Levant. Hence the Imperium Romanum subsumed, west-to-east, modern-day Portugal, Spain, England and France, Italy, Albania and Greece, the Balkans, Turkey, and parts of eastern, and southern Germany; southward it embraced parts of the Middle East: present day Syria, Lebanon, Israel, Jordan and more; thence southwestward it included the whole of ancient Egypt, then swept westward to contain the coastal regions of what are today Libya, Tunisia, Algeria, and Morocco, out to the longitudes just west of Gibraltar. Most of the people living there called themselves Romans, and lived under Roman law. Roman expansion began long before the state was changed into a monarchy and reached its zenith under Emperor Trajan with the conquest of Dacia (i.e., modern Romania and Moldova, as well as parts of Hungary, Bulgaria and Ukraine), in AD 106, and Mesopotamia in 116 (subsequently returned by Hadrian). At this territorial peak, the Roman Empire controlled approximately 5,900,000 km² (2,300,000 sq mi) of land surface, and so encompassed the Mediterranean Sea that the Romans called it mare nostrum - "our sea". Rome's influence upon the language, religion, architecture, philosophy, law and government of nations around the world lasts to this day.

The end of the Roman Empire is sometimes placed at 4 September 476 AD, when the last emperor of the Western Roman Empire, Romulus Augustus, was deposed and not replaced. Before this date, however, the Empire had been divided into Western and Eastern halves, Emperor Diocletian, who retired in 305, having been the last sole Emperor of an undivided Empire. The Western Roman Empire declined and fell apart (see Decline of the Roman Empire) in the course of the 5th century. The Eastern Roman Empire, known largely today as the Byzantine Empire, preserved Greco-Roman legal and cultural traditions along with Hellenic and Orthodox Christian elements for another millennium, until its eventual collapse with the conquest of Constantinople at the hands of the Ottoman Empire in 1453.