Explanation: we in the modern day differentiate between the Roman Republic and the Roman Empire. The Romans, however, regarded themselves as remaining a republic even with the Emperor at the head. There was a very strong taboo against the Emperor claiming to be king which would not be broken until the fall of the Western Empire. According to the Romans, the Emperor was just a very powerful magistrate who represented the will of the people, and who made decisions in consensus with the Senate.
Provincials sometimes did not appreciate this distinction - Greek cities would often write praises or requests to the Emperor, calling him 'Basileus' ('king'), and receive responses from Imperial secretaries thanking them for their praises or concerns - but insistently in the name of the 'autokrator' ('ruler'), not 'Basileus'.