The easy definition of an undocumented immigrant 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 popularly used jargon, nowhere found in the U.S. immigration laws. Because of its insulting connotations, however, we at Nolo prefer to use more neutral terms like "undocumented immigrant" or "unauthorized immigrant."
Estimates place the number of undocumented immigrants in the U.S. at around 11 million. Of course, it's an impossible number to know precisely, since it can change rapidly as people come and go, and because this is a population that attempts to stay under the radar.
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 from various countries, whether as tourists or using some other nonimmigrant (temporary) visa. Then they failed to leave after the expiration date of their permitted stay arrived.
Overstaying a visa carries its own consequences under U.S. immigration law.
The reason not all undocumented immigrants have been deported is partly because U.S. enforcement resources are insufficient to keep up with the numbers. Detaining and deporting even one person costs many thousands of dollars.
It's also important to understand that there are gray areas, where someone who might be viewed by some as undocumented has what almost amounts to a right to remain in the United States. 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, an asylum seeker who makes it into the U.S. unseen 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 might 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).
Some undocumented immigrants might also have the right to remain in the U.S. based upon a valid claim for Temporary Protected Status (TPS) or another form of immigration relief.
If you hire independent contractors, you're exempt from federal labor laws that require verifying everyone's lawful immigration status. (See Documentation Required to Work in the United States.) As soon as you become an employer however, even if it's just employing a nanny, you do fall under federal provisions making it illegal to hire an undocumented person. See, for example, Risks to Hiring an Undocumented Nanny.