Illegal means against the law. When something is illegal, a government has made a rule forbidding it, and people who do it anyway can face consequences like fines or even jail time.

Different places have different laws, so what's illegal in one country might be perfectly legal in another. In some countries, it's illegal to chew gum in public, while in others, that would seem ridiculous. Some activities are illegal almost everywhere because societies agree they cause serious harm, like stealing or assault.

The opposite of illegal is legal, which means allowed by law. Something can be legal without being good, and something can be illegal without being particularly harmful. For instance, jaywalking (crossing the street in the wrong place) is technically illegal in many cities, but it's a minor offense.

When you hear that something is illegal, it means a lawmaking body, like Congress or a city council, has officially prohibited it.