Old Church Slavonic (or Old Slavonic, or Old Slavic) is the first literary and liturgical Slavic language taken over (or developed, depending on the point of view) by the 9th century missionaries Saints Cyril and Methodius. It is important in Eastern Orthodoxy and in most countries of the Slavic peoples. (See also liturgical language.)
Cyril and Methodius hailed from Thessaloniki and based the Old Church Slavonic on the Slavic (Macedonian) dialect used by intellectuals of the Thessaloniki region of the Byzantine Empire. Bulgarian scholars consider Old Church Slavonic an Old Bulgarian dialect and call it Old Bulgarian. It has many South Slavic word forms.
A younger form of Old Church Slavonic is called Church Slavonic, but these terms are often confused.
Church Slavonic maintained a prestige status, particularly in Russia, for many centuries -- among Slavs in the East it had a status analogous to that of the Latin language in western Europe, but had the advantage of being less divergent from the vernacular tongues of average parishioners. Some Eastern Orthodox churches, such as the Russian and Serbian churches, still use Church Slavonic in their services and chants. Additionally, several Eastern Rite Catholic churches use Church Slavonic.