A foreign national is a person living in the U.S. who is not a citizen of the country in which he or she currently resides or, more generally, a person who is a citizen of any country other than the U.S.
It’s important to distinguish a foreign national from a nonresident alien, or an NRA, which is any person residing in the U.S. who is not a lawful permanent resident and who does not qualify for a green card or have the ability to pass a substantial presence test.
To be considered a foreign national, the person must have arrived on an approved carrier – assuming he or she arrives by air or sea – and can stay for no more than 90 days for pleasure, medical or business purposes. He or she must have a valid passport at all times.
Specific laws vary by country. Citizens of Mexico and Canada, for example, have specific policies, as do students.