Imperium

In ancient Rome, imperium was a form of authority held by a citizen to control a military or governmental entity. It is distinct from auctoritas and potestas, different and generally inferior types of power in the Roman Republic and Empire. One's imperium could be over a specific military unit, or it could be over a province or territory. Individuals given such power were referred to as curule magistrates or promagistrates. These included the curule aedile, the praetor, the consul, the magister equitum, and the dictator. In a general sense, imperium was the scope of someone's power, and could include anything, such as public office, commerce, political influence, or wealth.

Imperium originally meant absolute or kingly power—the word being derived from the Latin verb imperare (to command)—which became somewhat limited under the Republic by the collegiality of the republican magistrates and the right of appeal, or provocatio, on the part of citizens. Imperium remained absolute in the army, and the power of the imperator (army commander) to punish remained uncurtailed. The title imperator later was exclusively held by the emperor, as the commander of the armed forces. In fact, the Latin word imperator is the root of the English word emperor.

In ancient Rome, imperium could be used as a term indicating a characteristic of people, their wealth in property, or 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 (an imperator) had, in principle, absolute authority to apply the law within the scope of his magistracy or promagistracy. He could be vetoed or overruled either by a magistrate or promagistrate who was a colleague with equal power (e.g., a fellow consul), by one whose imperium outranked his – that is, one of imperium maius (greater imperium), or by a tribune of the plebs.

Some modern scholars such as A. H. M. Jones have defined imperium 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 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; any such magistrate was also escorted by lictors bearing the fasces (traditional symbols of imperium and authority), when outside the pomerium, axes being added to the fasces to indicate an imperial magistrate's power to inflict capital punishment outside Rome (the axes being 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 that of a praetor wore a sash ritually knotted on the front of his cuirass. Furthermore, any man executing imperium within his sphere of influence was entitled to the curule chair.

As can be seen, dictatorial imperium was superior to consular, consular to praetorian, and praetorian to aedilician; there is some historical dispute as to whether or not praetorian imperium was superior to "equine-magisterial" imperium. A promagistrate, or a man executing a curule office without actually holding that office, also possessed imperium in the same degree as the actual incumbents (i.e., proconsular imperium being more or less equal to consular imperium, propraetorian imperium to praetorian) and was attended by an equal number of lictors.

Certain extraordinary commissions, such as Pompey's famous command against the pirates, were invested with imperium maius, meaning they outranked all other holders of imperium of the same type or rank (in Pompey's case, even the consuls) within their sphere of command (his being "ultimate on the seas, and within 50 miles inland"). Imperium maius later became a hallmark of the Roman emperor.

Another technical use of the term in Roman law was for the power to extend the law beyond its mere interpretation, extending imperium from formal legislators under the ever-republican constitution: popular assemblies, senate, magistrates, emperor and their delegates to the jurisprudence of jurisconsults.