European Union vs EU

European Union and EU both are nouns.

Word	Noun	Adjective	Verb	Adverb
European Union	Yes	No	No	No
EU	Yes	No	No	No

As nouns, European Union and EU are synonyms defined as:

European Union and EU: an international organization of European countries formed after World War II to reduce trade barriers and increase cooperation among its members

Other synonyms of European Union include Common Market, EC, EEC, Europe, European Community, European Economic Community.

European Union (noun)	EU (noun)
an international organization of European countries formed after World War II to reduce trade barriers and increase cooperation among its members	an international organization of European countries formed after World War II to reduce trade barriers and increase cooperation among its members