NGS Style Manual‎ > ‎- U -‎ > ‎

undocumented immigrant, illegal immigrant, illegal immigration, illegal alien

undocumented immigrant: Use to describe an individual entering, living in, or working in the U.S. without legal permission. 

illegal immigrant: Avoid this term, except in a direct quote. 

illegal immigration: Acceptable when used to describe the issue rather than an individual.

illegal alien: Do not use.