The text on Bulgarian stamps
from the beginning of the 20th century (here Mi #112) translates to "Tsarstvo
Bulgaria". The ending "-stvo" means "-dom" like in "Kingdom" - or "Tsardom".
A direct translation would then lead to the title "Tsar" of the Bulgarian
rulers. However, considering the constitutional situation in the country,
it would be equivalent to use the title "King" in English. Thus, both titles
can be found in litterature.