It comes, via a long path that includes “kaiser”, from Caesar. One of the early Ivans started using it, it stuck, and even when they tried changing to imperato

It comes, via a long path that includes “kaiser”, from Caesar. One of the early Ivans started using it, it stuck, and even when they tried changing to imperator people kept using tsar/czar. In English it seems to be used informally to describe the person in charge even if the are more of a figurehead than a locus of actual authority.