Something that is Slavonic relates to East European languages such as Russian, Czech, and Serbo-Croat, or to the people who speak them.
The Ukrainians speak a Slavonic language similar to Russian.
...the department of Slavonic studies.
Slavonic in British English
(sləˈvɒnɪk) or especially US Slavic
noun
1.
a branch of the Indo-European family of languages, usually divided into three subbranches: South Slavonic (including Old Church Slavonic, Serbian, Croatian, Bulgarian, Bosnian, etc), East Slavonic (including Ukrainian, Russian, etc), and West Slavonic (including Polish, Czech, Slovak, etc)
2.
the unrecorded ancient language from which all of these languages developed
adjective
3.
of, denoting, or relating to this group of languages
4.
of, denoting, or relating to the people who speak these languages
Word origin
C17: from Medieval Latin Slavonicus, Sclavonicus, from Slavonia