A more thorough explanation:

Definition: The prefect of the city, also known as Praefectus Urbi, was a senator in ancient Rome who was responsible for maintaining law and order in the city of Rome. This position was created during the early Empire and the prefect had both criminal and civil jurisdiction. The prefect's civil jurisdiction was always cognitio, which means that they had the power to hear and decide cases.

Examples: The prefect of the city was originally appointed as a temporary delegate in case the emperor was absent. However, the position became permanent due to Tiberius' continued absence from Rome during the latter part of his reign. The prefect's duties included maintaining order in the city and commanding the urban cohorts, which was a police force consisting of 4,000 to 6,000 men. The prefect also had criminal jurisdiction and became the chief criminal court not only for Rome but also for the district within 100 miles.

Explanation: The examples illustrate how the prefect of the city was responsible for maintaining law and order in Rome. They had the power to hear and decide cases, command the police force, and act as the chief criminal court. The position became permanent due to the emperor's absence, and the prefect's duties expanded over time. This position was crucial in ensuring that Rome remained a safe and orderly city.