Impenetrable means impossible or extremely difficult to get through, understand, or see into. A thick jungle might be so overgrown with vines and thorns that it's impenetrable, meaning you can't force your way through it. A castle with stone walls three feet thick and no windows becomes an impenetrable fortress that enemies can't breach.

The word also describes things that are impossible to understand or figure out. When a textbook is written with dense, confusing language, students might call it impenetrable because they can't make sense of it no matter how hard they try. A person with an impenetrable expression keeps their feelings so hidden that you can't tell what they're thinking, even if you study their face carefully.

An impenetrable mystery is one that stumps even the best detectives. An impenetrable fog is so thick that you can't see through it at all. The word suggests complete resistance to entry, whether physical, visual, or mental.