To prohibit something means to formally forbid it or make it against the rules. When a school prohibits running in the hallways, students aren't allowed to do it. When a sign says “Swimming Prohibited,” it means swimming is not permitted.

The word carries official weight. Your parents might tell you not to eat cookies before dinner, but when a law prohibits something, breaking that rule can lead to serious consequences. For example, laws prohibit driving without a license, and city ordinances might prohibit skateboarding in certain areas.

Prohibition (the noun form) refers to the act of prohibiting something. The most famous example in American history is Prohibition with a capital P: from 1920 to 1933, the Constitution prohibited making, selling, or transporting alcohol throughout the United States. This period showed how difficult it can be to enforce a prohibition that many people disagree with.

You'll often see prohibit used in formal contexts: rules, laws, contracts, and official policies. A library might prohibit loud talking, a restaurant might prohibit outside food, or a game might prohibit certain moves. When something is prohibited, there is usually an authority behind the rule, someone with the power to enforce it and impose consequences for breaking it.