Yank vs Union soldier

Yank and Union soldier both are nouns.

Word	Noun	Adjective	Verb	Adverb
Yank	Yes	No	No	No
Union soldier	Yes	No	No	No

As nouns, Union soldier is a hyponym of Yank; that is, Union soldier is a word with a more specific, narrower meaning than Yank:

Yank: an American who lives in the North (especially during the American Civil War)
Union soldier: a member of the Union Army during the American Civil War

Other hyponyms of Yank include Federal, Federal soldier.

Yank (noun)	Union soldier (noun)
an American (especially to non-Americans)	a member of the Union Army during the American Civil War
an American who lives in the North (especially during the American Civil War)