An illegal act or technique.

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

An illegal immigrant.

Contrary to or forbidden by law, especially criminal law.

“Nearly 40 million people live in UK areas with illegal air pollution”

Forbidden by established rules.

“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.