A more thorough explanation:

Definition: Praefectus urbi (pronounced pri-fek-tus ur-bee) is a Latin term that means "prefect of the city." In Roman law, this was a senator who was responsible for maintaining law and order in the city of Rome. The position was created during the early Empire and the praefectus had both criminal and civil jurisdiction. The civil jurisdiction was gradually taken over from the praetor, although the praefectus's civil jurisdiction was always cognitio.

Examples: The praefectus urbi was responsible for maintaining order in the city of Rome and had under his command the urban cohorts, which was a police force numbering between 4,000 and 6,000 men. He also had criminal jurisdiction and became the chief criminal court not only for Rome but for the district within 100 miles.

Explanation: The examples illustrate the definition by showing the responsibilities and duties of the praefectus urbi. They were responsible for maintaining law and order in the city of Rome and had the power to enforce it through the urban cohorts. They also had criminal jurisdiction and were the chief criminal court for the city and surrounding district. This position was important in maintaining the stability and safety of the city during the Roman Empire.