adj

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

noun

• (obsolete) An illegal act or technique.
  "That whatſoever illegals hath been uſed againſt his friends and ſubjects, by impriſoning them without law, or impoſing fines upon their eſtates, be diſclaimed, and that perſons ſo committed be forthwith diſcharged."
• (colloquial, in the plural, as illegals) Contraband, esp. illegal substances such as drugs.
  "…I've got illegals in my bottom…!"
• (colloquial, offensive) An 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”."
• (espionage) A spy operating abroad illegally and under non-official cover, without visible ties to his or her country’s authorities.
  "Anna Chapman, whose glamorous appearance won her more publicity in the Western media than all the other illegals combined, was so successfully deceived by a US sting operation that she handed over her SVR laptop to an FBI agent posing as a Russian."