ancient Rome, one of the chief magistrates who governed in the absence of the kings, consuls, and emperors.

This power was greatest under the emperors. His chief care was the government of the city, taking cognizance of all crimes committed therein and within 100 miles. He judged capitally and finally, and even presided in the senate. He had the superintendence of the provisions, building, and navigation.

The prefect of modern Rome differs little from the ancient prefectus, his authority only extending to 40 miles round the city.

Prefect of the Praetorium, the leader of the praetorian bands destined for the emperor's guards, consisting, according to Dion, of 10,000 men. This officer, according to Suetonius, was instituted by Augustus, and usually taken from among the knights.

By the favour of the emperors his power grew very considerable; to reduce which, Constantine divided the prefecture of the praetorium into four prefectures, and each of these again he subdivided into civil and military departments, though the name was only referred to him who was invested with the civil authority, and that of comes belli given him who commanded the cohorts.