Old East Slavic, also referred to as
Old Russian (but not to be confused with the child dialects of the later post-Kievan Rus', Old Russian and
Old Ruthenian) and
Rusian (
sic, with one ‘s’, from
Rus’), was a language used in the 10th–15th centuries by
East Slavs in
Kievan Rus' and states which evolved after the collapse of Kievan Rus'.
Dialects of it were spoken, though not exclusively, in the area today occupied by
Belarus, central and northern
Ukraine, and western
Russia, and several eastern
voivodships of
Poland.