Rome had begun annexing provinces in the 3rd century BC, four centuries before reaching its greatest territorial extent, and in that sense was an "empire" while still governed as a republic. 

Beginning in the late 4th century, the Empire began to disintegrate as barbarians from the north overwhelmed the control of Christian Rome. Most chronologies place the end of the Western empire in 476, when Romulus Augustus was forced to abdicate to the Germanic warlord Odoacer. The empire in the East—known today as the Byzantine Empire, but referred to in its time as the "Roman Empire" or by various other names—ended in 1453 with the death of Constantine XI and the fall of Constantinople to the Ottoman Turks.

The Roman Empire was one of the largest in history, and the only empire to have encompassed territories in northern Europe, the Middle East, and North Africa. The Latin phrase imperium sine fine ("empire without end"  expressed the ideology that neither time nor space limited the Empire.