On this day 250 years ago in Philadelphia, the Continental Congress

Resolved, That in all continental commissions, and other instruments, where, heretofore, the words ‘United Colonies’ have been used, the stile be altered for the future to the “United States.”

Our country has been the United States of America ever since.

Sources: Journals of the Continental Congress, Vol. V accessed at https://archive.org/details/us_congress_continental/lljc005/page/747/mode/2up; https://constitutioncenter.org/blog/today-the-name-united-states-of-america-becomes-offici; https://www.smithsonianmag.com/history/how-did-the-nation-get-its-name-other-suggestions-included-the-united-colonies-columbia-and-the-united-states-of-alleghania-180989435/