A person who is not a US citizen and not a resident of the United States under the Green Card test or the substantial-presence test (e.g. 183 days or longer in the US). Those who meet one or more of these requirements are considered resident aliens.
Nonresident aliens are generally subject to US income tax only on their US-sourced income. To avoid backup withholding, nonresident aliens must file Form W-8BEN.
For information on determining residency status, see IRS Topic 851. For tax-planning ideas for foreign executives moving to the US to work, see an article elsewhere on this website.