Refers to a person who enters or stays in a country without legal permission. Often used in legal, news, or formal contexts.
An illegal immigrant was detained at the airport.
The government tightened controls on illegal immigrants.
不法滞在者 refers specifically to someone who overstays their visa or remains illegally after entry, while 不法入国者 emphasizes the act of entering illegally.
Compound of 不法 (illegal) + 入国 (entry into a country) + 者 (person).