As defined in the appropriate immigration regulations, an alien is deemed to be unlawfully present in the U.S. after the expiration of the period of authorized stay or is present in the U.S. without being admitted or paroled.
Note that no period of unlawful presence that accrued before April 1, 1997 may be considered for inadmissibility purposes.
Consequences of Unlawful Presence: The 3 and 10 Year Bars
3 Year Bar: From April 1, 1997 onwards, a person who has been in the U.S. in unlawful presence for a period of time between 180 days and less than 1 year consecutively, who then departs the U.S. and later attempts to re-enter, will face a bar to readmission for a period of 3 years. This 3 year period is calculated from the date of the persons departure from the county.
10 Year Bar: From April 1, 1997 onwards, if a person has been unlawfully present in the U.S. for 1 year consecutively or longer and then departs the country and later attempts to re-enter, they will be facing a 10 year bar to their readmission.