Russian is an East Slavic language of the wider Indo-European family. It is a descendant of the language used in Kievan Rus', a loose conglomerate of East Slavic tribes from the late 9th to the mid 13th centuries. From the point of view of spoken language, its closest relatives are Ukrainian, Belarusian, and Rusyn, the other three languages in the East Slavic branch. In many places in eastern and southern Ukraine and throughout Belarus, these languages are spoken interchangeably, and in certain areas traditional bilingualism resulted in language mixtures such as Surzhyk in eastern Ukraine and Trasianka in Belarus. An East Slavic Old Novgorod dialect, although it vanished during the 15th or 16th century, is sometimes considered to have played a significant role in the formation of modern Russian. Also Russian has notable lexical similarities with Bulgarian due to a common Church Slavonic influence on both languages, as well as because of later interaction in the 19th and 20th centuries, Bulgarian grammar differs markedly from Russian. In the 19th century (in Russia until 1917), the language was often called "Great Russian" to distinguish it from Belarusian, then called "White Russian" and Ukrainian, then called "Little Russian".
The vocabulary (mainly abstract and literary words), principles of word formations, and, to some extent, inflections and literary style of Russian have been also influenced by Church Slavonic, a developed and partly russified form of the South Slavic Old Church Slavonic language used by the Russian Orthodox Church. However, the East Slavic forms have tended to be used exclusively in the various dialects that are experiencing a rapid decline. In some cases, both the East Slavic and the Church Slavonic forms are in use, with many different meanings. For details, see Russian phonology and History of the Russian language.
Over the course of centuries, the vocabulary and literary style of Russian have also been influenced by Western and Central European languages such as Greek, Latin, Polish, Dutch, German, French, Italian, and English, and to a lesser extent the languages to the south and the east: Uralic, Turkic, Persian, and Arabic, as well as Hebrew.
According to the Defense Language Institute in Monterey, California, Russian is classified as a level III language in terms of learning difficulty for native English speakers, requiring approximately 1,100 hours of immersion instruction to achieve intermediate fluency. It is also regarded by the as a "hard target" language, due to both its difficulty to master for English speakers and its critical role in U.S. world policy.