It would seem that the answer is simple. The king and king are the successors of different empires. However, trying to figure out the origin of the word king, we are faced with a real etymological mystery.
According to written records, the word king has been known since 917. After Simeon's victory over the Byzantine army, he proclaimed himself king of the Bulgarians and Romans.
Ironically, the Russian word “tsar” comes from the Latin “caesar”, “caesar”. And ironically, because the first Caesar, who gave his name the title to all subsequent Roman emperors, was Gaius Julius Caesar, who did not want to be a king (in the Latin meaning of rex)! The fact is that the kings in Rome 500 years before the reign of Caesar were overthrown, and their very name was hated by the Romans. Following Gaius Julius, the rulers of both Rome and, later, Byzantium added to their name "Caesar" (Caesar) to emphasize their relationship with the great Caesar and called themselves emperors.
The official title of tsar in Russia appeared on January 16, 1547, after the wedding of Ivan IV, nicknamed the Terrible, who considered himself the heir to the Orthodox Byzantine Empire. Since Byzantium was the successor to the Roman Empire, its rulers were called Caesars.
And now about the king …
If we compare our purely Slavic name of the ruling person - prince, then it is more associated with knightly titles - bollard, Knight - in English.
The most common version of the origin of the word king goes back to the proper name of Charlemagne, whose empire at the beginning of the 9th century stretched from the Pyrenees to the Danube. However, when trying to find out the origin of the word king, we encountered a real etymological mystery.
Looking into the most voluminous etymological dictionary of the Russian language, the dictionary of Max Vasmer, (1996, v.2, p.333), we see that he deduces the etymology of the word king on behalf of Charlemagne. This opinion was shared not only by him, but at least 13 more linguists. However, the word king is present only in Slavic languages. In other languages, the word king sounds different. For example, in Latin - rex, in English - king, in German - knig, in French - roi.
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A reasonable question arises: what made Charles so famous in the Slavic lands that they inserted his name into their languages as a universal term denoting the ruler of the state? This is especially true of the Western Slavs, who called the king not only foreign, but also their rulers. Perhaps it makes sense to look for another etymology?
In fairness, it should be noted that in controversial cases such as this, Max Vasmer gives opinions with which he does not agree. One of them is the opinion of the Polish linguist Rudnitsky, who believes that the word king comes from the original Slavic word - to punish.
Emperor Akihito.
But a king or a king is bad who does not dream of becoming an emperor. As a rule, all those rulers whom it is now customary to call the Great were emperors, or became them. This is the first in history Emperor Octavian Augustus, who inherited the Roman Empire from Gaius Julius Caesar. ICarl the Great, after 9 centuries, who created an empire in the image and likeness of the Roman. And, finally, the Russian Peter the Great, who made the formidable Russian Empire out of a backward agrarian state.
In translation from Latin the word "emperor" means "ruler", "commander".
Currently, only the ruler of Japan, Emperor Akihito, has such a title, who, however, is only the titular ruler, while the prime minister has real power.
