Any person not a citizen or national of the United States.

U.S. Citizenship and Immigration Services. (2015). Glossary: alien. Retrieved from https://www.uscis.gov/tools/glossary.

(illegal) An alien who has entered the United States illegally and is deportable if apprehended, or an alien who entered the United States legally but who has fallen out of status and is deportable. Also known as an undocumentedalien. 

Internal Revenue Service. (2016). Immigration terms and definitions involving aliens: illegal alien. Retrieved from https://www.irs.gov/Individuals/International-Taxpayers/Immigration-Terms-and-Definitions-Involving-Aliens.