Impermeable means impossible for liquids or gases to pass through. An impermeable raincoat keeps you dry because water cannot soak through the fabric. An impermeable container holds gas without any leaking out.

Think of impermeable materials as having no tiny holes or gaps that would let things seep through. A plastic bag is impermeable to water. A rubber glove is impermeable to many kinds of paint and chemicals. Even your skin is mostly impermeable, which is why you don't absorb bathwater like a sponge.

Scientists and engineers care a lot about impermeability. They use impermeable clay to line landfills so garbage juice can't leak into groundwater. They design impermeable membranes for spacesuits so astronauts can breathe safely. Builders install impermeable barriers in basements to keep moisture from seeping through foundation walls.

The opposite is permeable, describing materials like coffee filters, sponges, or sandy soil that let liquids pass right through. When something is only partially impermeable, we might call it water-resistant rather than waterproof.