“The Cyrillic script is one of several scripts that were ultimately derived from the Greek script. The details of the history of that development and of the relationship between early forms of writing systems for Slavic languages has been lost. Cyrillic has traditionally been used for writing various Slavic languages, among which Russian is predominant. The earliest attestations of Cyrillic are for Old Church Slavonic manuscripts, dating to the 10th century ce. Old Church Slavonic is also commonly referred to as Old Church Slavic, and is abbreviated as OCS.
In the nineteenth and early twentieth centuries, Cyrillic was extended to write the non-Slavic minority languages of Russia and neighboring countries.“ - The Unicode Consortium, The Unicode Standard 15.0