Nero was Caesar, Commudus was Caesar, and Tiberius was Caesar.

They were all Caesar. Caesar was the term for the Emperor. I think they are asses, but that is just me.

I think it was Charlemagne who first coined the term Emperor. His father, Pepin the Short, was the one who shut the Mergovian empire down with a little letter to the pope.

But I think Caesar was the same as Emperor.