What is the term used to describe a citizen living in the us illegally?

This almost sounds like a trick question; if you are a citizen you can't be in the US illegally, and if you're in the US illegally you could not be a citizen!

That being said, if a person is residing in the US without a valid visa or residency permit, they would be classified as an illegal alien or the more politically correct term, "undocumented immigrant".