United States, United States of America, US, U.S., USA, U.S.A

The United States, or the United States of America in full, takes a singular verb because the term designates a single country rather than a collection of states. The expression these United States appears occasionally in American publications, but this plural form is not recommended in formal writing.

• The United States is a second home for many Canadians.
• Was the United States of America a signatory to the agreement?

The traditional practice is to use periods in the abbreviations for geographical names; and the government of the United States calls itself the U.S. Government (not US). However, there is a growing trend to drop the periods in such abbreviations. Thus, the abbreviations U.S. and US are both in common use as nouns or adjectives, while U.S.A. and USA are both used as nouns.

• The NAFTA trade agreement involves Canada, Mexico and the U.S.A
• The newspaper USA Today is popular outside U.S. borders.