In the broader sense the general military power of command of any (also non-Roman) commander; in the narrow sense, the military command of the highest officials in Rome (consul, praetor, dictator, magister equitum).
In the early Republic imperium is a partial aspect of the Roman power of office (auspicium). No later than the end of the 4th cent. BC do foreign policy conflicts with neighbours lead to the emphasis and stress on the military competence of the upper officials (auspicium imperiumque: Plaut. Amph. 192; 196). Finally in the late Republic, imperium denotes th…