EU vs European Union

EU and European Union both are nouns.

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

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

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

EU (noun)	European Union (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