INA §§212(a)(9)(C)(i)(I) & (II), 8 U.S.C. §§1182(a)(9)(C)(i)(I) & (II)
This ground of inadmissibility is a bit complicated to understand, and often trips up immigration attorneys. There are basically two aspects to this rule.
First, the person must have been unlawfully present in the United States for a year or longer and then left the country. Alternatively, the person could have been ordered removed (deported) from the United States, and then left.
Second, the person must later actually or attempt to enter the U.S. illegally. This means either illegally crossing back into the U.S., or even just trying to do so.
A person who fits the requirements above is permanently inadmissible to the United States. There is a partial waiver available, but the individual must wait 10 years before she can apply.