The U.S. Immigration and Nationality Act (I.N.A.) contains a number of grounds of "inadmissibility," including one called the "permanent bar." Broadly speaking, these grounds of inadmissibility are barriers to U.S. entry based on things like criminal and security violations, health troubles, likelihood of requiring need-based government assistance, and immigration violations.
In literal terms, someone who is inadmissible to the U.S. will be denied any request for an immigrant visa (green card) or in many cases a temporary (nonimmigrant) visa.
The permanent bar comes from Section 212(a)(9)(C)(i) of the I.N.A., which makes inadmissible "Any alien who (I) has been unlawfully present in the United States for an aggregate period of more than 1 year, or (II) has been ordered removed under section 235(b)(1) , section 240 , or any other provision of law, and who enters or attempts to reenter the United States without being admitted." This part of the law came from legislative changes made in 1996, called the Illegal Immigration Reform and Immigrant Responsibility Act or IIRIRA.
For example, let's say someone crossed the U.S. border from Mexico illegally, stayed for a year, went home for a while, and then tried crossing the U.S. border a second time. That second attempt (whether it succeeded or not) makes the person inadmissible under this section. Leaving and coming back a few times so that no stay is longer than one year would not have helped, either. The word "aggregate" in the law means that it's enough that the person's stay added up to one year in total.
Similarly, someone who was removed from the U.S. and then tries to enter without going through the required admission procedures will be subject to the permanent bar.
The first thing to understand in unpacking this part of the law is what "unlawful presence" means. In general, it refers to someone who either entered the U.S. illegally (such as by crossing the border secretly, or stowing away in a boat or car); or who came to the U.S. legally (on a visa or the Visa Waiver Program) and then failed to leave on time.
Lawyers have tried to argue that the same unlawful-presence exceptions apply to the permanent bar as to a related grounds of inadmissibility known as the three- and ten-year bars, such as for minors or people with an asylum application awaiting USCIS action. So far, however, these arguments have failed (but check with a lawyer in your own jurisdiction for the latest.)
The only available exceptions at this time have to do with timing, given that this section of the law went into effect on April 1, 1997. First, people who last reentered the U.S. before April 1, 1997 after a previous deportation are not subject to the permanent bar. Second, people who accrued unlawful presence before April 1, 1997 need not count this time towards the aggregate one year needed to trigger the permanent bar.
Many grounds of inadmissibility can be waived, or automatically expire after a set period of time. But the permanent bar is harder, though not impossible, to overcome.
The statute says that someone subject to the permanent bar can request permission to enter the U.S. after waiting ten years first. The government interprets this to mean ten years outside the United States. But after these ten years, you can't just go ahead and submit a visa or green card application by the usual methods. You must first obtain the U.S. government's "consent" to your application.
To request this consent for a green card application, you must use a form called "Permission to Reapply for Admission" or I-212, issued by U.S. Citizenship and Immigration Services (USCIS). For a nonimmigrant visa, you'll need to request an exercise of discretion.
There also a waiver to the permanent bar available to VAWA self-petitioners who can show a connection between their having been battered or subjected to extreme cruelty and their removal, departure from the U.S., reentry or reentries into the United States, or attempts at reentry.
Other, more limited exceptions also apply in unusual cases.
If you think you might be eligible for a visa or green card, but are potentially affected by the permanent bar, definitely consult an experienced immigration lawyer. The lawyer can confirm whether you are subject to this bar and help develop a strategy for overcoming this in your quest for a visa or green card.