ETYM Pref. il- not + legal: cf. French illégal.
Prohibited by law or by official or accepted rules.
amerciable · banned · black · black-market · bootleg · contraband · criminal · dirty · embezzled · extrajudicial · extralegal · felonious · hot · ill-gotten · illegitimate · illicit · ineligible · irregular · misappropriated · misbranded · mislabeled · nonlegal · outlaw · outlawed · penal · prohibited · punishable · smuggled · under-the-counter · unlawful · unratified
Not allowed, or leading to invalid results. For example, an illegal character in a word processing program would be one that the program cannot recognize; an illegal operation might be impossible for a program or system because of built-in constraints. Compare invalid.
An illegal immigrant.