In Ancient Rome, imperium was the degree of authority vested in a curule magistrate or promagistrate. It meant that a man literally owned the authority of his office, and could not be overruled provided he was acting within the limits of his level of imperium and within the laws which governed his conduct.
Imperium was conferred by a lex curiata, and lasted for only one year. Extensions for a prorogued governor had to be ratified by the Senate and/or the People. Lictors shouldering fasces indicated a man's imperium; the more lictors a man had, the higher his imperium.