Dictionary

An illegal act or technique.

(plural, as illegals) Contraband, esp. illegal substances such as drugs.

An illegal immigrant.

A spy working abroad illegally and undercover, without visible ties to his or her country’s authorities.

Contrary to or forbidden by law, especially criminal law.

Example: Nearly 40 million people live in UK areas with illegal air pollution

Forbidden by established rules.

Example: Moving a pawn backward is an illegal move in chess.

(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.

Example: illegal immigrant;  illegal logger;  illegal pilot