North vs Northern United States

North and Northern United States both are nouns.

Word	Noun	Adjective	Verb	Adverb
North	Yes	No	No	No
Northern United States	Yes	No	No	No

As nouns, North and Northern United States are synonyms defined as:

North and Northern United States: the region of the United States lying to the north of the Mason-Dixon line

North (noun)	Northern United States (noun)
the United States (especially the northern states during the American Civil War)	the region of the United States lying to the north of the Mason-Dixon line
the region of the United States lying to the north of the Mason-Dixon line
British statesman under George III whose policies led to rebellion in the American colonies (1732-1792)