Imperium in a broad sense translates as power. In ancient Rome the concept applied to people, and meant something like "power status" or "authority", or could be used with a geographical connotation and meant something like "territory". It is not to be mistaken with auctoritas ("authority").

Personal characteristic

In ancient Rome, imperium could be used as a term indicating a characteristic of people, the measure of formal power they had. This qualification could be used in a rather loose context (for example, poets used it, not necessarily writing about state officials). However, in Roman society it was also a more formal concept of legal authority. A man with imperium had in principle absolute authority to apply the law within the scope of his magistracy or promagistracy, but could be vetoed or overruled by a magistrate or promagistrate having imperium maius (a higher degree of imperium) or, as most republican magistratures were multiple (though not quite collegial since each could act on his own), by the equal power of his colleague (e.g., the other consul). Some modern scholars, such as A.H.M. Jones have defined it as "the power vested by the state in a person to do what he considers to be in the best interests of the state".

Imperium can be distinguished from regnum, or royal power, which was inherited. Imperium was originally a military concept, the power of the imperator (general in the army) to command. The word derives from the Latin verb, imperare ("to command"). The title imperator was applied to the emperor, who was the commander of the armed forces. In fact, the Latin word, imperator, gives us the English word emperor.

Imperium was indicated in two prominent ways. A curule magistrate or promagistrate carried an ivory baton surmounted by an eagle as his personal symbol of office (compare the field marshal's baton). Any such magistrate was also escorted by lictors bearing the fasces (traditional symbols of imperium and authority); when outside the pomerium, axes were added to the fasces to indicate an imperial magistrate's power to enact capital punishment outside of Rome (the axes were removed within the pomerium). The number of lictors in attendance upon a magistrate was an overt indication of the degree of imperium. When in the field, a curule magistrate possessing an imperium greater or equal to praetorian imperium wore a sash ritually knotted on the front of his cuirass. Further, any man executing imperium within his sphere of influence was entitled to the curule chair.

• Curule Aedile (aedilis curulis)- 2 lictors
• Since a plebeian aedile (aedilis plebis) did not own imperium, he was not escorted by lictors.
• Consul- 12 lictors each
• Magister equitum (the dictator's deputy)- 6 lictors
• Praetor- 6 lictors (2 lictors within the pomerium)
• Dictator- 24 lictors outside the Pomerium and 12 inside; starting from the dictatorate of Lucius Sulla the latter rule was ignored.
• Because the dictator could enact capital punishment within Rome as well as without, his lictors did not remove the axes from their fasces within the pomerium.