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EuroVoc is a multilingual thesaurus (controlled vocabulary) maintained by the Publications Office of the European Union and hosted on the portal Europa.
EuroVoc defines Central and Eastern Europe at
https://op.europa.eu/en/web/eu-vocabularies/concept/-/resource?uri=http://eurovoc.europa.eu/914
with the following 21 countries:
Albania, Belarus, Bosnia and Herzegovina, Bulgaria,
Caucasus countries: Armenia, Azerbaijan, Georgia;
Croatia, Czechia, Hungary, Kosovo, Moldova, Montenegro,
North Macedonia, Poland, Romania, Russia,
Serbia, Slovakia, Slovenia, Ukraine