Unbreachable means impossible to break through, penetrate, or overcome. When something is unbreachable, it forms such a strong barrier that nothing can get past it.

A castle's walls might seem unbreachable to an attacking army. A computer security expert might design an unbreachable firewall to keep hackers out. Scientists working with dangerous materials use unbreachable containment systems to prevent any leaks.

You might hear about an unbreachable contract (one with no loopholes), an unbreachable defense in football (impossible to get past), or an unbreachable alliance between friends (a bond so strong nothing could break it).

In practice, truly unbreachable things are rare. What seems unbreachable today might not stay that way forever. Medieval castle builders thought their walls were unbreachable until cannons were invented. The word can also describe something extremely difficult to breach rather than literally impossible, though people use it to emphasize just how secure or solid something is.