USA vs billion

USA and billion both are nouns.

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

USA (noun)	billion (noun)
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 9 zeros
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	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
	a very large indefinite number (usually hyperbole)

USA (adjective)	billion (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