- Adjective. (legal) Contrary to or forbidden by law, ''especially'' criminal law.
- Adjective. Not permitted by rules.
- Adjective. (philately, of an issue printed for collectors) Totally fictitious, and ''often'' issued on behalf of a non-existent territory or country.
- Adjective. (context, of an immigrant) Being an illegal immigrant; residing in a country illegally.
- Noun. (colloquial, derogatory) An illegal immigrant.