a name, Caesar.

More context because I know too much about the Roman empire. A gross simplification follows below.

While Caesar was the name of the dictator for life Julius and his adopted heir and biological grand nephew Octavian later Augustus. It became more of a title as Rome entered its middle Imperial era. Each emperor in the jedioclaudian dynasty was in theory a member of the Caesar line, and therefore was addressed by that name. In time, the emperor of the day would take the title Augustus and typically designate an heir as Caesar, continuing the legal fiction of legitimacy dating back to the Caesar Augustus. This is definitely seen after the Crisis of the Third Century, where the empire was decided up into east and west with both halves having a senior emperor Augustus and a respective junior emperor Caesar.

Skipping ahead a thousand years... With the fall of Constantinople in 1453 to the Ottomans, Sophia Palaiologina, niece of the last Eastern Roman emperor, ended up married to Ivan III, the prince of Moscow. Ivan then claimed the title of Tsar (Ceaser) through right of marriage. His grandson Ivan IV (the terrible) went on to claim the title of Tsar of all Russia.

I won't go into the history of the Tsars because that's not my wheelhouse.