Franchise context: The write-up below is from the parent franchise article (anime, book series, or IP) rather than the card game itself. A dedicated CCG profile has not been written yet.
The title of imperator was originally the rough equivalent of commander under the Roman Republic. Later, it became a part of the titulature of the Roman emperors as their praenomen. The Roman emperors 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 and the later Roman Empire. It was abbreviated to "IMP" in inscriptions. The word is an agentive form of the verb imperare, meaning "to order, to command". The English word emperor derives from imperator via French.
Source: en.wikipedia.org/wiki/Imperator