billion vs USA

billion and USA both are nouns.

Word	Noun	Adjective	Verb	Adverb
billion	Yes	Yes	No	No
USA	Yes	No	No	No

billion (noun)	USA (noun)
the number that is represented as a one followed by 9 zeros	the army of the United States of America; the agency that organizes and trains soldiers for land warfare
the number that is represented as a one followed by 12 zeros; in the United Kingdom the usage followed in the United States is frequently seen	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 very large indefinite number (usually hyperbole)

billion (adjective)	USA (adjective)
denoting a quantity consisting of one thousand million items or units in the United States
denoting a quantity consisting of one million million items or units in Great Britain