|This article does not cite any references or sources. (November 2008)|
Rus' (Old East Slavic русьскаꙗ землꙗ "land of the Rusĭ (Русь)") is an ethno-cultural region in Eastern Europe inhabited by Eastern Slavs. Historically, it comprises the northern part of Ukraine, the north-western part of Russia, Belarus and some neighbouring parts of Finland, the Baltic states, Poland and Slovakia.
The name comes from Old East Slavic Русь (Rus’), and remains the same in modern Belarusian, Russian and Ukrainian. In Greek it is called Ρως (Ros) or Ρωσία (Rosia), in Armenian Ռուսաստան (Rrusastan), and in Latin Ruthenia or Russia. For more about the etymology, see Rus (name).
The Kievan Rus had its cultural and political height in the 10th and 11th century and later disintegrated into many small principalities. The most powerful successor states were Vladimir-Suzdal, Halych-Volhynia and Novgorod Republic. The Mongol invasion of Rus' devastated the north-eastern parts in 1237–1238 and the south western parts of Rus in 1239–1240. From this date on the fate of Rus lands began to diverge. The north-eastern parts stayed under the rule of Mongols and the Golden Horde for two more centuries while the western parts together with Kiev were conquered by Lithuania (see Battle on the Irpen' River) and Poland. This political development caused a certain cultural and linguistic split between Eastern Slavs which previously almost didn't exist. However, the Orthodox church which resided in Vladimir and later in Moscow remained a common unifying factor of Rus lands. In the Grand Duchy of Lithuania which had an Eastern Slavic majority of population as well as the official Slavic language the Orthodox Christians started to face discrimination by the ruling Lithuanian elite in the second half of the 15th century. A rivalry between the Grand Duchies of Moscow and Lithuania both of whom considered themselves as centers of Rus and legitimate successors of Kiev (Ivan III called himself the Grand Duke of all Rus) caused several wars and showed Muscovite predominance by the beginning of the 16th century. Lithuania was forced to seek help from Poland and the two states formed the Polish-Lithuanian Commonwealth where Poland began to play a dominant role promoting Catholic rule in the Western Rus.
Russia managed to gradually roll back its opponent by the end of 18th century when Catherine the Great initiated the Partitions of Poland. By that time, all Rus lands were united under control of Saint-Petersburg, except Galicia which became part of Austria-Hungary.
Ethnonym Рус(ъ)(c)(ь)кие 
People of different principalities, be it in the East or in the West, continued to call themselves Русские (Русъские,Руськие, Русины, Руси(ч)ци) which meant belonging to Rus. The languages were also called Руски, Руськый, Русский while the entirety of the region was called Русская земля, Rusish land or Rusland.
The Greeks used to call the lands around Kiev Rossia Mikra (Rus proper) and the entirety of all other lands Rossia Makra (Rus in the broader sense, Greater Rus). Later, this coined the terms Great Russia and Little Russia.