1. a foreigner who has entered or resides in a country unlawfully or without the country's authorization.
2. a foreigner who enters the U.S. without an entry or immigrant visa, esp. a person who crosses the border by avoiding inspection or who overstays the period of time allowed as a visitor, tourist, or businessperson. Cf. resident alien. Also called ille'gal im'migrant.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.