The Roman Empire was the post-Republican phase of the government of the city of Rome on Old Earth. It continued the expansionist policies of the Roman Republic, and adopted much of the civilization (mathematics, literature, theatre, architecture, etc) of the Eastern End of the Mediterranean Sea after conquering it. The Western part of the Roman Empire had collapsed as a political entity by about 450CE, while the Eastern part of the Roman Empire survived in one form or another until about 1450CE.
The Roman Empire contributed enduring themes to later civilizations. Perhaps most important ws the idea of a universal or nearly so, unitary state, revived in the Holy Roman Empire in Western Europe around 800 CE. And the idea of a universal code of law, largely in the form of the Justinian Code. Several religions from that area became important in Roman life, most notably Mithraism (especially popular in the Army) and Christianity (at first mostly in the lower and slave classes). Christianity survived well into Post Diaspora times in various forms. (HH5)
- Detailed article about the Roman Empire on Wikipedia