"Yankee" is derogatory or at best playful. "Usonians" isn't a word or in the form of a word.
"American" or "America" in this context is short for "United States of America" and has been used that way for generations, here and around the world. For the geographical term, use "North America," etc.; for the cultural, perhaps use "Anglo-America" or "Latin America."
"American" or "America" in this context is short for "United States of America" and has been used that way for generations, here and around the world. For the geographical term, use "North America," etc.; for the cultural, perhaps use "Anglo-America" or "Latin America."