Union Army vs army

Union Army and army both are nouns.

Word	Noun	Adjective	Verb	Adverb
Union Army	Yes	No	No	No
army	Yes	No	No	No

As nouns, army is a hypernym of Union Army; that is, army is a word with a broader meaning than Union Army:

Union Army: the northern army during the American Civil War
army: a permanent organization of the military land forces of a nation or state

Other hypernyms of Union Army include ground forces, regular army, blue.

Union Army (noun)	army (noun)
the northern army during the American Civil War	a permanent organization of the military land forces of a nation or state
	a large number of people united for some specific purpose