A nonresident alien is an individual who is not a U.S. citizen or U.S. national and who does not meet either the "green card" or "substantial presence" test as described in IRS Publication 519, U.S. Tax Guide for Aliens. The "green card" test refers to lawful permanent residents of the U.S. who have been given the privilege, according to the immigration laws, of residing permanently as an immigrant. The "substantial presence" test refers to the number of days a nonresident alien is physically present in the U.S. during a certain period of time. If a person does not meet either of these tests, they are classified as a nonresident alien. Nonresident aliens are taxed only on their income from sources within the U.S. and on certain income connected with the conduct of a trade or business in the U.S. .