What is the definition of tsar?

Answer 1

Tsar, czar, kaiser, are all derivatives of Caesar the Roman title. As the Romans left their conquered territories as the Roman Empire declined, they left behind people who assumed leadership and took the title which was adapted to their own language.

Answer 2:

The above is absolutely right - Tsar/Czar and Kaiser are all derivatives of Caesar, the title of the Emperors of ancient Rome. However, the Romans never conquered Russia, or much of Germany. The rulers of those countries, and others, took titles that linked to Caesar to identify with the Roman Empire. It was a title very distinguished, and later monarchs copied it.