adjective
Contrary to, forbidden, or not authorized by law, especially criminal law.
Forbidden by established rules.
(philately, of an issue printed for collectors) Totally fictitious, and often issued on behalf of a non-existent territory or country.
(of a person, sometimes offensive) Being or doing something illegally.
(chiefly US, sometimes offensive) Being an illegal immigrant; residing in a country illegally.
noun
(obsolete) An illegal act or technique.
(colloquial, in the plural, as illegals) Contraband, esp. illegal substances such as drugs.
(colloquial, offensive) An illegal immigrant.
(espionage) A spy operating abroad illegally and under non-official cover, without visible ties to his or her country’s authorities.