The development of the Bulgarian language may be divided into several historical periods. The prehistoric period (essentially late Common Slavonic) occurred between the Slavonic invasion of the eastern Balkans and the mission of St. Cyril and St. Methodius to Great Moravia in the 860s. Old Church Slavonic (9th to 11th century, also referred to as Old Bulgarian), a literary norm of the early southern dialect of Common Slavonic from which Bulgarian evolved, was the language used by St. Cyril, St. Methodius and their disciples to translate the Bible and other liturgical literature from Greek. Middle Bulgarian (12th to 15th century) was a language of rich literary activity and major innovations. Modern Bulgarian dates from the 16th century onwards; the present-day written language was standardized on the basis of the 19th-century Bulgarian vernacular. The historical development of the Bulgarian language can be described as a transition from a highly synthetic language (Old Bulgarian) to a typical analytic language (Modern Bulgarian) with Middle Bulgarian as a midpoint in this transition.
Fewer than 20 words remain in Bulgarian from the language of the Bulgars, the Central Asian people who moved into present-day Bulgaria and eventually adopted the local Slavic language. The Bolgar language, a member of the Turkic language family or the Iranian language family (Pamir languages), is otherwise unrelated to Bulgarian.
Old Church Slavonic was the first Slavic language attested in writing. As Slavic linguistic unity lasted into late antiquity, in the oldest manuscripts this language was initially referred to as языкъ словяньскъ, "the Slavic language". In the Middle Bulgarian period this name was gradually replaced by the name языкъ блъгарьскъ, the "Bulgarian language". In some cases, the name языкъ блъгарьскъ was used not only with regard to the contemporary Middle Bulgarian language of the copyist but also to the period of Old Bulgarian. A most notable example of anachronism is the Service of St. Cyril from Skopje (Скопски миней), a 13th century Middle Bulgarian manuscript from northern Macedonia according to which St. Cyril preached with "Bulgarian" books among the Moravian Slavs. The first mention of the language as the "Bulgarian language" instead of the "Slavonic language" comes in the work of the Greek clergy of the Bulgarian Archbishopric of Ohrid in the 11th century, for example in the Greek hagiography of Saint Clement of Ohrid by Theophylact of Ohrid (late 11th century).
During the Middle Bulgarian period, the language underwent dramatic changes, losing the Old Slavonic case system, but preserving the rich verb system (while the development was exactly the opposite in most other Slavic languages) and developing a definite article. It was influenced by its non-Slavic neighbours in the Balkan linguistic union (mostly grammatically) and later also by Turkish, which was the official language of Ottoman empire, in the form of the Ottoman language (an earlier form of Turkish), mostly lexically. As a national revival occurred towards the end of the period of Ottoman rule (mostly during the 19th century), a modern Bulgarian literary language gradually emerged which drew heavily on Russian and Church Slavonic/Old Bulgarian and which later reduced the number of Turkish and other Balkanic loans. Today one difference between Bulgarian dialects in the country and literary spoken Bulgarian is the significant presence of Russian or Church Slavonic words and even word forms in the latter. The phonology of many such words has been modified along Bulgarian patterns; many other words were taken from Russian without taking the expected phonetic changes in consideration (оборот, непонятен, ядро and others).Modern Bulgarian was based essentially on the Eastern dialects of the language, but its pronunciation is in many respects a compromise between East and West Bulgarian.