IMPĔRĬUM, was under the republic a power, without which no military
operation could be carried on as in the name and on the behalf of the
state. It was not incident to any office, and was always specially
conferred by a lex curiata, that is, a lex passed in the comitia
curiata. Consequently, not even a consul could act as commander of
an army, unless he were empowered by a lex curiata. It could not be
held or exercised within the city in the republican period; but it
was sometimes conferred specially upon an individual for the day
of his triumph within the city, and at least, in some cases, by a
plebiscitum. As opposed to _potestas, imperium_ is the power which
was conferred by the state upon an individual who was appointed to
command an army. The phrases _consularis potestas_ and _consulare
imperium_ might both be properly used; but the expression _tribunitia
potestas_ only could be used, as the tribuni never received the
imperium. In respect of his imperium, he who received it was styled
_imperator_. After a victory it was usual for the soldiers to salute
their commander as imperator, but this salutation neither gave nor
confirmed the title, since the title as a matter of course was given
with the imperium. Under the republic the title came properly after
the name; thus Cicero, when he was proconsul in Cilicia, could
properly style himself M. Tullius Cicero Imperator, for the term
merely expressed that he had the imperium. The emperors Tiberius and
Claudius refused to assume the praenomen of imperator, but the use
of it as a praenomen became established among their successors. The
term imperium was applied in the republican period to express the
sovereignty of the Roman state. Thus Gaul is said by Cicero to have
come under the imperium and ditio of the populus Romanus.