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.

Altostslawisch, auch Altrussisch bezeichnet eine ca. zwischen dem 10. und dem 14. Jahrhundert v. a. in der Kiewer Rus benutzte Sprache, die Vorgängerin der heutigen ostslawischen Sprachen.