billionth vs U.S.

billionth and U.S. both are nouns.

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

billionth (noun)	U.S. (noun)
one part in a billion equal parts	the executive and legislative and judicial branches of the federal government of the United States
position 1,000,000,000 in a countable series of things	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

billionth (adjective)	U.S. (adjective)
the ordinal number of one billion in counting order