forbidden by law or statute.

contrary to or forbidden by official rules, regulations, etc.

The referee ruled that it was an illegal forward pass.

Informal . illegal alien.

adjective

allowed , authorized , ethical , good , lawful , legal , legitimate , moral , permissible , right

adjective

actionable , banned , black-market , bootleg * , contraband , criminal , crooked , extralegal , felonious , forbidden , heavy * , hot * , illegitimate , illicit , interdicted , irregular , lawless , not approved , not legal , outlawed , outside the law , prohibited , proscribed , prosecutable , racket , shady , smuggled , sub rosa , taboo , unauthorized , unconstitutional , under the table , unlawful , unlicensed , unofficial , unwarrantable , unwarranted , verboten , violating , wildcat * , wrongfulnotes:a form of conduct will be illicit and a type of practice will be illegal , wrongful , bootlegged , misbegotten , nugatory , ultra vires