Airtight means sealed so completely that no air can get in or out. An airtight container keeps chips crunchy and prevents cookies from going stale because moisture in the air can't reach the food inside. Scientists use airtight chambers when they need to control exactly what gases are present in an experiment.

The word also describes an argument or plan that's so well thought out that it has no weaknesses. When a detective presents an airtight case in court, the evidence is so strong and complete that there's no reasonable way to question it. If your excuse for being late is airtight, it's so convincing and verifiable that no one can doubt it.

Think of the difference between a regular container and an airtight one: a regular lid might keep things mostly fresh, but an airtight seal guarantees nothing gets through. Similarly, an airtight argument doesn't just sound good: it accounts for every objection someone might raise. When lawyers say a contract is airtight, they mean it's written so carefully that no one can find a loophole or way around it.