Aka the Russian Empire and before that, Czarist Russia ("Czar" or "Tsar" being a Slavic form of "Caesar", this title also existed in medieval Bulgaria and Serbia, but was most historically important in Russia). Massive in size (sometimes bigger than even the USSR was) and lasted for about 400 years. The Muscovite Tsardom began under the 15th-century grand prince Ivan III "The Great" (who used the tsar title only occasionally) and was established fully under his grandson, Ivan IV "The Terrible", who was crowned as a Tsar from the very beginning. It was a convoluted, very conservative realm that considered itself a successor state to the Byzantine Empire.