Originally, the name Rus (Русь, Rus’) referred to the people, the region, and the medieval states (9th to 12th centuries) of the Rus' Khaganate and Ruthenia (Kievan Rus) polities. In the western cultures and English language the term is better known as Ruthenia since 11th century. Its territories are today distributed among Belarus, Ukraine, and a part of the European section of Russian Federation.
One of the earliest written sources mentioning the people called Rus (as Rhos) dates back to year 839 AD in a Royal Frankish chronicle Annales Bertiniani; the Frankish authorities identified them as a Germanic tribe called Swedes. According to the Kievan Rus Primary Chronicle, compiled in about 1113 AD, the Rus were a group of Varangians, Norsemen who had relocated somewhere from the Baltic region (literally "from beyond the sea"), first to Northeastern Europe, then to the south where they created the medieval Kievan state.
The modern name of Russia (Rossiya), which came into use in the 17th century, is derived from the Greek Ρωσσία (nowadays spelled Ρωσία) which in turn derives from Ρως, an early Greek name for the people of Rus'. "Rus'" as a state had no proper name; by its inhabitants it was called "rusĭska zemlya".(русьска(я) земля) (with rusĭska alternatively spelled russka, rus'ka, and most often ruska), which might be translated as "Land of the Rus". The word "rusĭska" is an adjective: the morpheme -ĭsk- is used to form adjectives in Slavic; -a is a grammatical ending for feminine adjectives (namely, zemlya, "land", is grammatically feminine in Slavic). In similar fashion, Poland is called Polska by its inhabitants, that is, Pol-sk-a, originally being the adjective Polish (land).
To distinguish the medieval "Rus" state from other states that derived from it, modern historiography calls it "Kievan Rus'." Its predecessor, the 9th-century "Rus' Khaganate," is a somewhat hypothetical state whose existence is inferred from a handful of early medieval Byzantine and Persian/Arabic sources that mention that the Rus' people were governed by a khagan.