Russian is a Slavic language of the Indo-European family. It is a lineal descendant of the language used in Kievan Rus'. From the point of view of the spoken language, its closest relatives are Ukrainian, Belarusian, and Rusyn, the other three languages in the East Slavic group. 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, e.g. Surzhyk in eastern Ukraine and Trasianka in Belarus. An East Slavic Old Novgorod dialect, although 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–20th centuries, although Bulgarian grammar differs markedly from Russian. In the 19th century, the language was often called "Great Russian" to distinguish it from Belarusian, then called "White Russian" and Ukrainian, then called "Little Russian".
What are Russian's closest relatives?
Ukrainian, Belarusian, and Rusyn