illegal
adj.adj. against the law. You use this to describe an action or thing that the government or a formal rule does not allow.
adj. contrary to or forbidden by law, especially criminal law. Often used to describe activities, substances, or statuses that violate official statutes.
It is illegal to park your car in front of a fire hydrant.
The company faced heavy fines after the government discovered they were dumping illegal waste into the local river.
While the moral implications were debated for years, the high court eventually ruled that the surveillance program was illegal under the current constitution.
Borrowed from French illégal, from Medieval Latin illegalis, from Latin legalis, by surface analysis, il- + legal. In senses relating to immigration, via clipping of illegal alien or illegal immigrant.
Typically used as a predicative adjective after 'is' or 'was', or as an attributive adjective before a noun.
He did an illegalHe did something illegalIn standard English, illegal is an adjective and cannot be used as a noun to mean a crime; it must modify a noun like 'act' or 'activity'.