Monster Hunter Imperium refers to the commission given to the PC's owner to hunt troublesome monsters in Roman controlled territories. The word "Imperium" more or less means, "Commission".
"In ancient Rome, imperium could be used as a term indicating a characteristic of people, the wealth held in items, 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 ("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) or by one whose imperium outranked his - that is, one of imperium maius (greater imperium).
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 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 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 that of a praetor 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.
Magister equitum (the dictator's deputy) � 6 lictors
Praetor � 6 lictors (2 lictors within the pomerium)
Consul � 12 lictors each
Dictator � 24 lictors outside the Pomerium and 12 inside; starting from the dictatorship 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.
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 owners of imperium (in Pompey's case, even the consuls) within their sphere of command (his being "ultimate on the seas, and within 50 miles inland")."