The Western Roman Empire refers to the western half of the Roman Empire. (The other half is the Byzantine Empire or Eastern Roman Empire.) Administrative division of the sprawling Empire into a western and eastern half with co-emperors for each began under Diocletian in 285 and was periodically abolished and recreated for the next two centuries until final abolishment by the Byzantine emperor Zeno in 480. By that time there was little effective control left in the Western Empire.

A Western Roman Empire existed intermittently in several periods between the 3rd and 5th centuries, after Diocletian's Tetrarchy and the reunifications associated with Constantine the Great and Julian the Apostate (324–363). Theodosius I divided the Empire upon his death (in 395) between his two sons. Eighty-five years later, Zeno would recognize the reality of the Western Empire's reduced domain (Imperial control had been lost over even the Italian Peninsula) after the death Western Emperor Julius Nepos and rule as sole emperor.

The Roman Empire (Latin: IMPERIVM ROMANVM) was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean.

The 500-year-old Roman Republic, which preceded it, had been weakened and subverted through several civil wars. Several events are commonly proposed to mark the transition from Republic to Empire, including Julius Caesar's appointment as perpetual dictator (44 BC), the Battle of Actium (2 September 31 BC), and the Roman Senate's granting to Octavian the honorific Augustus (16 January 27 BC).

Roman expansion began in the days of the Republic, but the Empire reached its greatest extent under Emperor Trajan: during his reign (98 to 117 AD) the Roman Empire controlled approximately 6.5 million km² of land surface. Because of the 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, architecture, philosophy, law, and forms of government in the territory it governed, particularly Europe, and by means of European expansionism throughout the modern world.