The easy definition of an undocumented immigrant is that he or she is a foreign-born person who doesn’t have a legal right to be or remain in the United States. But that’s where the easy part stops. Let’s look at some common questions and misconceptions about undocumented immigrants.
Theoretically yes, but “illegal alien” is not a technical term–it’s the popularly used jargon, nowhere found in the U.S. immigration laws. Because of its insulting connotations, however, we at Nolo prefer to use the more neutral term, “undocumented immigrant,” or "unauthorized immigrant."
Some, but not all. Although accurate numbers are hard to come by, it’s clear that a significant number of undocumented immigrants originally came to the U.S. legally, whether as tourists or on some other nonimmigrant (temporary) visa, and then failed to leave.
Partly because U.S. enforcement resources are insufficient to keep up with the numbers. Back in 2011, an Immigration and Customs Enforcement (ICE) director estimated that it would cost an average of $12,500 to deport each person. Those costs may well have risen then, given the Trump Administration's "zero-tolerance" policy, under which most illegal entrants are prosecuted and placed into detention—which is estimated to cost up to $200 per adult per day, with average stays of around six weeks.
It’s also important to understand that there are gray areas, where someone who might be considered undocumented has what almost amounts to a right to remain in the United States. Some undocumented immigrants are unaware that they might have the right to remain in the U.S. based upon a valid claim for asylum, Temporary Protected Status (TPS), or another form of immigration relief.
For example, asylum seekers being sought for persecution by their own government, who are unable to make direct contact with a refugee agency in their own country, might have no choice for U.S. entry but to come illegally. If stopped at the border, they should be allowed to state their claim and, if their fear is found credible, to see an immigration judge.
Or, if an asylum seeker makes it into the U.S. unseen, he or she has one year in which to prepare an application for asylum. Although the person has no actual right to be in the U.S. during that time, a would-be applicant who is caught and placed into removal proceedings can claim asylum at that point, and may very well be approved for asylum (and one year later, be eligible for a green card).
What's more, under a policy called “prosecutorial discretion,” various immigrants, such as students and those with close family members in the U.S., are supposed to be largely left alone by the immigration authorities, so that the authorities can concentrate on immigrants who are criminals or otherwise a risk to U.S. society. Some of these immigrants may actually be granted a sort of limbo status called "deferred action," and in some cases a work permit. See, for example, Deferred Action for Childhood Arrivals (DACA) (still active owing to court decisions, though the Trump Administration has tried to undo it).