Meaning the “illustrious” or “exalted” one in Latin, Augustus was a special name granted to Octavian Caesar by the Roman Senate after he assumed power. Later emperors used “Augustus” as a title. By about the third century, it came to refer to the senior emperor in the same way as “Caesar” was used as a title for a junior co-emperor or the imperial heir. Following Diocletian’s political reforms, the two senior emperors were both called Augusti.

Caesar was the surname of Julius Caesar, who was the great-uncle and adopted father of the first emperor, Augustus. In life, Julius Caesar made himself dictator for life over Rome and was assassinated. “Caesar” became a title used by emperors, even the ones who reigned after the downfall of Nero who were biologically unrelated to Julius Caesar and Augustus. Over time, “Caesar” was the title used by the junior co-emperor or the designated heir of the reigning emperor. After Diocletian reformed the imperial system, the two junior co-emperors were called the Caesars.

Before the emperors, the two consuls were the executive office of the republic, much like a president or a prime minister. However, there were always two consuls in office sharing power, and their terms were usually limited to a year.