Imperium ("complete power", "military command", "order") is a specific, military aspect and form of executive power within Nova Roma which does not allow place for any procedure of refusal, negation, or disobedience. Imperium is one of the most specifically Roman legal terms which define the Roman constitutional thinking, and it is most faithfully and authentically reconstructed in Nova Roma, as well, according to the limitations and practicalities of our modern circumstances. In ancient Rome, imperium meant complete and unlimited power to give orders, outside of the city of Rome, to soldiers and citizens alike, which was natural for a Roman mind, because all citizens were considered part of the Roman people's army, if not active soldiers, at least as reservists. Inside the city of Rome, imperium was limited and magistrates who possessed it could not use it, but they had to rely on democratic and administrative measures. In our Nova Roma, imperium works the same way: within the central, international administration of Nova Roma, imperium may not be exercised it its unlimited form. Nova Roman magistrates and promagistrates with imperium exercise it within their provinces only. Imperium is always accompanied with auspicia maiora, the right to take the highest auspices of the state in order to convene the comitia or the senate, or to establish new institutions and to start (ceremonial) military initiatives.

Legal definition of imperium

Imperium in Nova Roma is regulated by the lex Cornelia Domitia de re publica constituenda and the edictum consulare de imperio et potestate. Imperium is defined by the lex Cornelia Domitia as chief command and presidency, complete power over provinces and military, with supreme executive, judicial and disciplinary power, which includes the following rights:

• the ius imperandi: the power to command and to give orders to anyone,
• full iurisdictio: the power to interpret and administer the law and to administer justic,
• full ius coercitionis: the right to enforce the law and their own decisions, even by disciplinary or preventive actions,
• ius intercessionis: the right to forbid decisions or actions of the colleague or lesser magistrates, and
• ius edicendi: the right to issue edicta.
• the right of having lictors with fasces as a symbol of supreme power: 12 for consuls and proconsuls, 6 for praetors and propraetors, 5 for legati pro praetore.

All of these rights can be exercised in an unlimited way only within the boundaries or provincia of the office, in the central administration it is subject to both provocatio and tribunician intercessio. Only the dictator and the magister equitum can exercise imperium within the central administration of Nova Roma.

Imperium is distinct from potestas which is the power and rights invested in one's office, granted to the officer in order to execute the duties of the office. There are certain offices which include the same rights, encoded in the office, which is also included in imperium. For example, a consul, as part of his consular office, is granted the power ius coercitionis, ius intercessionis and iurisdictio to administer Nova Roma, however, the rights of ius coercitionis, ius intercessionis and iurisdictio are also within his imperium as an addition. This is how a proconsul retains the rights of ius coercitionis, ius intercessionis and iurisdictio within his province even if his consular office has expired: because his imperium, which includes ius coercitionis, ius intercessionis and iurisdictio, is renewed for the province, the proconsul retains his ius coercitionis, ius intercessionis and iurisdictio, not because of the title "proconsul" or because of holding a governorship, but because of being invested with imperium.

Investment with imperium

Imperium is granted by the comitia curiata, and no magistrate may exercise the rights contained in imperium and auspicia maiora without the formal grant by the comitia curiata. When imperium is prorogued, it does not have to be invested again by the comitia curiata, but it automatically continues to be in force. No governor may assume imperium and auspicia without a taking of office ceremony of the religio Romana before, including an inauguration and taking the vows of imperium, the oaths and auspices for the term of office with favorable result. A governor without these procedural steps may not legally exercise any powers of his office. When a governor's imperium is prorogued, none of these ceremonies are required, but they are required when the imperium of a sitting magistrate is prorogued for the first time.

Degrees of imperium

Consuls and proconsuls (or any commissioner receiving imperium pro consule) of Nova Roma have full or greater imperium, praetors and propraetors (or any commissioner receiving imperium pro praetore) have a lesser degree of imperium. Those possessing consular imperium can give orders to those possessing only praetorian imperium.

Previous uses and definitions of imperium in Nova Roma

Before the enactment of the lex Lucilia de re publica restituenda on a.d. VII Kal. Sept. ‡Q. Arrio (III) A. Tullia cos. ‡ MMDCCLXXIV a.u.c., imperium had a much more direct and powerful use in Nova Roma, granting even more power to the magistrates possessing it, granting an unhistorically wide range of rights. Before the lex Lucilia, imperium was regulated by the lex Arminia de imperio: all officers granted imperium had the right to call the senate and the comitia to order, except those whose powers were limited to provincial affairs; and they had ius coercitionis even within the central administration, and, in general, the use of imperium was not explicitly excluded from the central administration, although the ius provocationis and tribunician intercessio were in force under those years, too, and they put a limit on the use of imperium in that previous period of Nova Roma, as well. Due to this difference in the definition of imperium, before the repeal of the lex Arminia de imperio, the special proconsul S. Lucilius Tutor who was granted full consular imperium, without territorial restriction, could call, fully legitimately, the comitia to enact the lex Lucilia de re publica restituenda. In today's terminology, which would be the more accurate terminology, as well, both S. Lucilius Tutor, and Cn. Cornelius Lentulus Alexander before him, had full not only full consular imperium, but full consular potestas, since according to the lex Arminia de imperio they had all powers of a consul.