1. the lands and peoples subject to the authority of ancient Rome.
2. the form of government established in ancient Rome in 27b.c.,comprising the Principate or Early Empire (27b.c.–a.d.284) and the Autocracy or Later Empire (a.d.284–476).
3. a later empire, as that of Charlemagne or the Byzantine Empire, regarded as a restoration or continuation of the ancient Roman Empire or one of its branches. Cf. Holy Roman Empire.