U.S. vs American

U.S. and American both are nouns.

Word	Noun	Adjective	Verb	Adverb
U.S.	Yes	No	No	No
American	Yes	Yes	No	No

U.S. (noun)	American (noun)
the executive and legislative and judicial branches of the federal government of the United States	the English language as used in the United States
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	a native or inhabitant of the United States
	a native or inhabitant of a North American or Central American or South American country

U.S. (adjective)	American (adjective)
	of or relating to or characteristic of the continents and islands of the Americas
	of or relating to the United States of America or its people or language or culture