Ivan IV. The Terrible.
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"Tsar" is quite another word.' As it is habitually used with respect to the kings of Kazan, Astrakhan, and Siberia, and as Ivan IV. (Vassilievich) seems to have adopted it, more particularly after the conquest of these two neighboring kingdoms, Huppel thinks that it came thence, and that the Russian autocrats, after having gained this considerable extension to their territory, assumed the title of the vanquished sovereigns."
see more - Historic Summary - Early Annals
Sears, Robert. An Illustrated Description of the Russian Empire. New York: Robert Sears, 1855