Impervious means impossible to pass through or penetrate. A raincoat made of rubber is impervious to water: rain hits it and slides right off instead of soaking through to your clothes. A metal safe is impervious to a burglar's crowbar.

It describes anything that forms a complete barrier. Scientists might talk about rock layers that are impervious to water, which is why underground lakes can form. Builders use impervious materials like concrete and plastic to keep moisture out of foundations.

The word also describes people who seem immune to criticism, emotions, or influence. A student who remains impervious to peer pressure doesn't change their behavior just because others pressure them. A judge should be impervious to bribes and favoritism, making decisions based only on facts and law. When someone seems impervious to reason, they won't listen to logical arguments.

Being impervious can be admirable, like staying impervious to fear during a crisis, or problematic, like being impervious to others' feelings. The key idea is that nothing gets through, whether it's water, influence, or emotion. An impervious barrier holds firm.