"Tsar" and its variants were the official titles of the following states:
The title Tsar is derived from the Latin title for the Roman emperors, Caesar. In comparison to the corresponding Latin word "imperator", the Byzantine Greek term basileus was used differently depending on whether it was in a contemporary political context or in a historical or Biblical context. In the history of the Greek language, basileus had originally meant something like "potentate". It gradually approached the meaning of "king" in the Hellenistic Period, and it came to designate "emperor" after the inception in the Roman Empire. As a consequence, Byzantine sources continued to call the Biblical and ancient kings "basileus" even when that word had come to mean "emperor" when referring to contemporary monarchs (while it was never applied to Western European kings, whose title was transliterated from Latin "rex" as ῥήξ, or to other monarchs, for whom designations such as ἄρχων "leader", "chieftain" were used).
As the Greek "basileus" was consistently rendered as "tsar" in Slavonic translations of Greek texts, the dual meaning was transferred into Church Slavonic. Thus, "tsar" was not only used as an equivalent of Latin "imperator" (in reference to the rulers of the Byzantine Empire, the Holy Roman Empire and to native rulers) but was also used to refer to Biblical rulers and ancient kings.
From this ambiguity, the development has moved in different directions in the different Slavic languages. Thus, the Bulgarian language and Russian language no longer use tsar as an equivalent of the term emperor/imperator as it exists in the West European (Latin) tradition. Currently, the term tsar refers to native sovereigns, ancient and Biblical rulers, as well as monarchs in fairy tales and the like. The title of king (Russian korol' , Bulgarian kral) is sometimes perceived as alien and is by some Russian-speakers reserved for (West) European royalty (and, by extension, for those modern monarchs outside of Europe whose titles are translated as king in English, roi in French etc.). Foreign monarchs of imperial status, both inside and outside of Europe, ancient as well as modern, are generally called imperator (император), rather than tsar.