USA is an abbreviation for the United States of America. US tends to be the more popular way to abbreviate United States. Both of these abbreviations can serve as either adjectives or nouns. However, some style guides prefer United States as the noun form and US as the adjective form.
USA is a noun because United States of America is a noun. A noun is a person, place, animal, thing, or idea. USA functions as a noun, for example, in Bruce Springsteen’s song lyric, “I was born in the USA.”
US and U.S.
US and U.S. are both abbreviations of United States. Both of these abbreviations can be correct. The difference between them (whether to use periods or not) is just a matter of style.
The Associated Press Stylebook prefers the abbreviation U.S. with periods. However, the Chicago Manual of Style prefers US without periods. To complicate matters, the Chicago Manual of Style allows U.S. in some situations. The Associated Press Stylebook (which is mostly used by newspapers) advises writers to use US in headlines.
US as an Adjective
The Chicago Manual of Style advises writers to use US as an adjective. For example, “He grew up in New Zealand, but he loves US television shows.” In this case, US is an adjective because it modifies television shows.
According to the Chicago Manual of Style, the abbreviation US shouldn’t be used as a noun. Instead, the full phrase United States should be used when the text requires a noun. For example, “He loves US culture, so he decided to move to the United States.” In this example, US acts as an adjective, while the unabbreviated United States acts as a noun.
The Associated Press takes the same approach to this debate. It advises writers to use U.S. when the text requires an adjective and United States when the text requires a noun.
The abbreviation USA is a noun, but the abbreviations U.S. and US are preferred by most style guides. Some style guides advise writers to use the abbreviations only as adjectives, and to use United States when a noun is required. However, other style guides allow US to be both an adjective and a noun. Whatever style guide you’re using (or even if you aren’t using one), the most important thing is to stay consistent throughout your work.