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Mere undocumented presence in the United States alone, however, in the absence of a previous removal order and unauthorized reentry, is not a crime under federal law.

Being in the U.S. without documentation is considered a civil matter, said Nancy Morawetz, professor of clinical law at New York University School of Law. "Being present in the U.S., that status, is not a crime," Morawetz said.

That doesn’t mean undocumented immigrants can live in the U.S. without consequence, said Rick Su, a professor at the University at Buffalo School of Law. Federal authorities can deport them.

A criminal violation comes with a punishment, like time in prison. Civil cases come with penalties instead. Deportation is considered a penalty under federal law, not a punishment.

"Congress can decide what they want to make a crime and what they want to make a civil violation," Su said. "What they’ve decided is that immigration violations by themselves are civil violations."

There are some immigration laws that can land an immigrant in criminal court. Entering the U.S. illegally is a crime, for example, but staying after a temporary visa has expired is a civil violation