The officer in charge of a Roman camp varied and his title was not set during the historical period. His title depended upon what kind of magistrate was commissioned--in Republican times, usually by the Senate--for the commanding role. This couild be a (sitting) praetor or consul or a former praetor or consul (i.e., a propraetor or proconsul). Sometimes, especially under the Empire, he was known as a 'praefectus'. But the commander of a specific field army ws generally known as an 'imperator', as he held the 'imperium' ('power of command') which was conferred upon him by some civil authority or, in Imperial times, often by the army itself.