Quote Originally Posted by Baba Ga'on
The original title meant a position below that of basileus, the Byzantine Greek term for emperor (though it meant king, once upon a time). I'm sure it meant more to Bulgarian and Serbian rulers than did their simple, native titles (khan, king, etc.), but in the end it originally meant nothing more than a status as friend and subordinate of the emperor in Constantinople -- which was, to the Byzantines, of course, more than good enough for some Slavic savage, even if he had defeated them.
Basileus is more properly translated as soveregin. As originally it refered more to the person than his office. And Aristotle exposed that a Basileus was, what we would call a constitutional monarch (restrained by law). While a Tyrant was would be an absolute monarch.

Quote Originally Posted by Baba Ga'on
Also, note that Peter the Great took on the title emperor, which stood above his titles of tsarship.
Actually it replaced several of his titles of Tsarship. Peter changed his fathers title of Czar & Grand Prince, Autocrat of all Greater, Lesser & White Russia,

of Moscow, Kiev, Vladimir, Novgorod


to

Emperor & Autocrat of all Russia,

of Moscow, Kiev, Vladimir, Novgorod,


But that was just the primary line of a very long list of titles. One of which was "Dominator of the northern countries."

Quote Originally Posted by Baba Ga'on
See above. What's important here is to remember that things had changed in the time period between Diocletian and the Byzantine court of the 9th century (or was it the 8th?).
In some ways yes and some ways no. The Slavs took the title of Tsar because they weren't Romans. And thus couldn't be crowned emperor.