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.