Generally, the term "unlawful presence" in the United States refers to time spent in the United States after an unlawful entry or visa overstay.
The Immigration and Nationality Act (INA) punishes people who have stayed in the United States unlawfully, or accumulated unlawful presence in the United States, by barring those persons from re-entering the United States if they leave and attempt to return legally. The length of the person's bar depends on the length of time he/she remained unlawfully in the United States.
In general, the time bars for unlawful presence in the United States are as follows:
* If you accrue unlawful presence of more than 180 continuous days but less than one year, but you leave before any official, formal removal procedures (i.e. deportation) are begun against you, you will be barred from reentering the United States for a period of three years.
* If you accrue unlawful presence of more than 365 continuous days, then leave prior to any deportation or other formal procedures being started against you, you will be subsequently barred from reentering the United States for a period of ten years.
* If you accrue unlawful presence of more than one year total (in the aggregate, not necessarily continuous), or are ordered removed (deported) from the U.S., and then subsequently enter or attempt to enter without inspection (for example, are smuggled across the border), you will be permanently barred from the U.S., -- possibly for your entire life, though after ten years, you can request special permission to apply for a visa or green card.