Old Church Slavic is a highly important language that was widely used in many parts of Eastern Europe during medieval times. It's known for its complex grammar, rich vocabulary, and unique script. While "Old Church Slavic" is the most commonly used term to describe the language, there are several other synonyms that can be used as well. Some of these include Old Bulgarian, Old Slavonic, and Old Church Slavonic. These synonyms are used interchangeably to refer to the same language, and are important for scholars and linguists who are interested in studying the history and evolution of Eastern European languages and cultures.