Final answer:
The middle division of the Roman Empire is the High/Middle Empire, which lasted for about 100 years between the reign of Trajan and the assassination of the last Antonine emperor. It was a period of great geographical expansion and relative peace known as the Pax Romana.
Step-by-step explanation:
The middle division of the three historical divisions of the Roman Empire is the High/Middle Empire. During the approximately 100 years between the reign of Trajan and the assassination of the last Antonine emperor, Comodus (98-192 CE), the Roman Empire reached its greatest geographical extent. This period is known as the High Empire and was characterized by relative peace and prosperity, referred to as the Pax Romana.