USA vs Union

USA and Union both are nouns.

Word	Noun	Adjective	Verb	Adverb
USA	Yes	No	No	No
Union	Yes	Yes	No	No

USA (noun)	Union (noun)
the army of the United States of America; the agency that organizes and trains soldiers for land warfare	the United States (especially the northern states during the American Civil War)
North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776

USA (adjective)	Union (adjective)
	being of or having to do with the northern United States and those loyal to the Union during the American Civil War