eastern United States vs USA

eastern United States and USA both are nouns.

Word	Noun	Adjective	Verb	Adverb
eastern United States	Yes	No	No	No
USA	Yes	No	No	No

eastern United States (noun)	USA (noun)
the region of the United States lying to the north of the Ohio River and to the east of the Mississippi River	the army of the United States of America; the agency that organizes and trains soldiers for land warfare
	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