Something illegal is against the law or breaks the rules. If you're reading this in jail, you've probably done something illegal, and if you're not in jail, there's plenty of time to obey the law.

You can use the adjective illegal to describe breaking the rules, as when you head-butt someone in a game. Acts that go against the law, such as robbing a bank, are also illegal. There is a wide range of things called illegal, from small acts to big ones, but no matter the seriousness, if it is against the law, it is illegal. This adjective also describes people who enter countries without official government approval, they are called "illegal immigrants" or "illegal aliens."

Definitions of illegal

adj prohibited by law or by official or accepted rules

“an illegal chess move”
contrary to or prohibited by or defiant of law
of a crime or misdemeanor; punishable by a fine set by a judge
banned, prohibited
forbidden by law
black, black-market, bootleg, contraband, smuggled
distributed or sold illicitly
criminal, felonious
involving or being or having the nature of a crime
dirty, ill-gotten
obtained illegally or by improper means
embezzled, misappropriated
taken for your own use in violation of a trust
beyond the usual course of legal proceedings; legally unwarranted
extralegal, nonlegal
not regulated or sanctioned by law
recently stolen or smuggled
illegitimate, illicit, outlaw, outlawed, unlawful
contrary to or forbidden by law
prohibited by official rules
misbranded, mislabeled
branded or labeled falsely and in violation of statutory requirements
penal, punishable
subject to punishment by law
done or sold illicitly and secretly;
lacking legal authority
of marriages and offspring; not recognized as lawful
contrary to rule or accepted order or general practice
established by or founded upon law or official or accepted rules
conformable to or allowed by law
ordered by a court of law
decreed by or proceeding from a court of justice
jural, juristic
of or relating to law or to legal rights and obligations
lawful, legitimate, licit
authorized, sanctioned by, or in accordance with law
ratified, sanctioned
formally approved and invested with legal authority
prescribed or authorized by or punishable under a statute
sub judice
before a judge or court of law; awaiting judicial determination
of marriages and offspring; recognized as lawful
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