Proper noun
Old East Slavic
(historical) A Slavic language used from the 10th to the 14th centuries by East Slavs in the state of Kievan Rus and its successors. The ancestor of Belarusian, Russian, Rusyn and Ukrainian.
At the same time, most legal documents throughout Rus' were written in a purely Old East Slavic language (supposed to be based on the Kiev dialect of that epoch). Source: Internet
Hilarion's discourse on Yaroslav and his father Vladimir is frequently cited as the first work of Old East Slavic literature. Source: Internet