The Latin word imperator derives from the stem of the verb imperare, meaning ‘to order, to command’. It was originally employed as a title roughly equivalent to commander under the Roman Republic. Later it became a part of the titulature of the Roman Emperors as part of their cognomen. The English word emperor derives from imperator via Old FrenchEmpereür. The Roman emperors themselves generally based their authority on multiple titles and positions, rather than preferring any single title. Nevertheless, imperator was used relatively consistently as an element of a Roman ruler's title throughout the principate (derived from princeps, from which prince in English is derived) and the dominate. In Latin, the feminine form of imperator is imperatrix, denoting a ruling female.
English words such as imperial derive from this Latin word.
When Rome was ruled by kings, to be able to rule, the king had to be invested with the full regal authority and power. So, after the comitia curiata, held to elect the king, the king also had to be conferred the imperium.