The Roman Emperors were monarchial rulers of the Roman State during the imperial period (from about 27 BC onward). The Romans had no single term for the office: Latin titles such as imperator (from which English Emperor derives), augustus, caesar and princeps were all associated with it. In practice, the Emperor was supreme ruler of Rome and supreme commander of the Roman legions. In theory, however, Rome remained a republic, the res publica, and the Emperor's status was merely that of primus inter pares - first among equals. This legal fiction became increasingly meaningless as the Emperors consolidated their power. However, it was maintained at least to a ceremonial degree until the very end of the Roman Empire - 476 in the Western Roman Empire and 1453 in the East.
There was no constitutional office of "Roman Emperor", nor any title or rank directly analogous to the title of "Emperor"; all the titles traditionally associated with the Emperor had pre-existing, Republican meanings.