Anyone who is not a native of the United States of America.
The term "American" fails to accurately describe someone living in the USA, because "America" refers to all of North and South America, so unless you are refering to each and every person on these two continents, you can not use the term "American". When refering to someone who lives in the USA, they are called a United Stater. Click tater hater for more information.
"Wow, George Bush is such a United Stater! So am I! So is everyone who lives in the USA!"
United Staters are Americans, but Americans are not always United Staters.