Το άτομο δηλαδή που για κάποιο χρονικό διάστημα δεν είχε χαρακτηριστεί ως μόνιμος κάτοικος. Ένα άτομο που π.χ. πήγε με σύμβαση 3 μηνών στις ΗΠΑ είναι nonresident alien .. μετά τους 3 μήνες όμως έμεινε για μόνιμη εργασία και έκανε αίτηση για πράσινη κάρτα άρα η νομική του κατάστση άλλαξε από nonresident σε resident.

You are a dual status alien when you have been both a resident alien and a nonresident alien in the same tax year. Dual status does not refer to your citizenship, only to your resident status for tax purposes in the United States. In determining your U.S. income tax liability for a dual-status tax year, different rules apply for the part of the year you are a resident of the United States and the part of the year you are a nonresident. The most common dual-status tax years are the years of arrival and departure.
http://www.irs.gov/businesses/small/international/article/0,...


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Note added at 17 mins (2007-06-11 09:22:45 GMT)
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And from Wikipedia:

n U.S. law, an alien is a person who owes political allegiance to another country or government and not a native or naturalized citizen of the land where they are found.[1] Types of "alien" persons are:

* An alien who is legally permitted to remain in a country which is foreign to him or her. On specified terms, this kind of alien may be called a legal alien of that country.
* An alien who has temporary or permanent residence in a country (which is foreign to him/her) may be called a resident alien of that country.
* A visitor with the legal right to visit a country (which is foreign to him/her) may be called a nonresident alien of that country.
* The term illegal alien describes a foreign national who resides in another country unlawfully, either by entering that country at a place other than a designated port-of-entry or as result of the expiration of a non-immigrant visa. See illegal immigration for greater detail.
* An enemy alien is an alien who is designated as an enemy; compare to enemy combatant.

http://en.wikipedia.org/wiki/Alien_(law)

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Note added at 48 mins (2007-06-11 09:53:15 GMT)
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Είναι νομική η κατάσταση του ατόμου. Για να γίνεις resident πρέπει να έχεις κάνει και έναρξη διαδικασίας για πράσινη κάρτα. Ειδάλλως, υπό οιαδήποτε άλλη κατάσταση (είτε τουρίστας είτε με Βίζα εργασίας) θεωρείσαι Non resident.

Επίσης ας σημειωθεί ότι σας δίνω πηγές αντί να σας πω το "10 χρόνια έμενα Αμερική και πλήρωνα φόρους" .... πέρασα από όλα τα στάδια του residency. Έλεος πια!