Kalmar "Tsohon Bulgarian" tana nufin farkon nau'in yaren Bulgariya, wanda aka yi amfani da shi tun daga ƙarni na 9 zuwa na 11. Ana kuma san shi da Old Church Slavonic ko Old Slavonic, kuma shi ne yaren da ake amfani da shi don dalilai na liturgical a cikin Cocin Orthodox. Tsohon harshen Bulgarian ya dogara ne akan yarukan Slavic da ake magana da su a yankin, kuma Girkanci da Latin sun rinjaye shi sosai. An dauke shi a matsayin harshen adabin Slavic na farko, kuma ya taka muhimmiyar rawa wajen bunkasa al'adar rubutacciyar Slavic.