The ancient
Romans, to begin with, didn't have much love for
kings and
emperors; even though their
Republic was becoming more and more corrupt, they clung to their ideals. That's why when Augustus siezed power and became
Rome's first true emperor, he did not call himself
rex or
imperator - the people would never have had it - but instead Caesar, after his predecessor, Julius. This established a healthy tradition of giving proper respect without actually mentioning the whole "emperor" thing. Its exact political meaning shifted over time, but by the end, Caesar = Emperor.
That's why today we have in German Kaiser and in Russian Czar, both of which mean emperor.