Definition of "illegal" in English

adjective

1. Contrary to, forbidden, or not authorized by law, especially criminal law.

   • This is illegal, you know!
   • Nearly 40 million people live in UK areas with illegal air pollution
   • Their illegal mining operations in Asia, Africa and elsewhere, are protected by officials and police paid to look the other way – and powerful customers in the construction industry who prefer not to ask too many questions.

2. Forbidden by established rules.

   • Moving a pawn backward is an illegal move in chess.
   • The computer program performed an illegal operation and was terminated.

3. (philately, of an issue printed for collectors) Totally fictitious, and often issued on behalf of a non-existent territory or country.

4. (of a person, sometimes offensive) Being or doing something illegally.

   • illegal immigrant
   • illegal logger
   • illegal pilot

5. (chiefly US, sometimes offensive) Being an illegal immigrant; residing in a country illegally.

noun

1. (obsolete) An illegal act or technique.

2. (colloquial, in the plural, as illegals) Contraband, esp. illegal substances such as drugs.

3. (colloquial, offensive) An illegal immigrant.

4. (espionage) A spy operating abroad illegally and under non-official cover, without visible ties to his or her country’s authorities.