You can use the adjective illegally to describe breaking the rules, such as hitting someone`s head in a game. Acts that violate the law, such as robbing a bank, are also illegal. There is a wide range of things that are labeled illegal, from small acts to big ones, but no matter how serious the law, if it breaks the law, it is illegal. This adjective also describes people who enter countries without official government authorization, they are called “illegal immigrants” or “illegal aliens”. Something illegal is against the law or breaks the rules. If you`re reading this in prison, you`ve probably done something illegal, and if you`re not in prison, you have plenty of time to obey the law.