Wiki has a pretty interesting history on that [1]:
During its existence, Kievan Rus was known as rusĭskaja zemlja, translated as the "land of Rus" or the "Rus land" (Old East Slavic: роу́сьскаꙗ землꙗ́), with Rus being derived from the ethnonym Роусь. ... The first known use of the term Kievan Rus comes from the 1582 chronicle by Polish historian Maciej Stryjkowski, where it was used to describe the region of Rus state immediately adjacent to Kyiv.
In 1837 it was adopted by Kyiv University rector Mykhailo Maksymovych in relation to the Principality of Kiev; known for his Ukrainophile political orientation, Maksymovych later spread that definition to the whole realm, seeing Kyiv as the original point where Rus statehood emerged, establishing the base for the future Russian Empire.