illegal /ɪˈliːɡəl/adjective 5noun 4forms 5 adjective5 •against the lawThis is illegal, you know!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. •Forbidden by established rules.Moving a pawn backward is an illegal move in chess.The computer program performed an illegal operation and was terminated. •(offensive) Being or doing something illegally. •(US, offensive) Being an illegal immigrant; residing in a country illegally. •Totally fictitious, and often issued on behalf of a non-existent territory or country. noun4 •A spy operating abroad illegally and under non-official cover, without visible ties to his or her country’s authorities. •(informal, offensive) Ellipsis of illegal immigrant.In Manchester and in Aldershot in Hampshire, asylum seeker accommodation was targeted by demonstrators carrying placards that read “deport them, don’t support them” and “no apartments for illegals”. — 2024 •[in the plural] (informal) Contraband, esp. illegal substances such as drugs.…I've got illegals in my bottom…! — 2004 •(no longer used) An illegal act or technique. formsillegals · illegaler · more illegal · illegalest · most illegal