If something is impermeable, liquid or gas cannot pass through it. Scientists use it for soil, rock, or a layer that water cannot soak through; rain then sits on top. A coat or a bag can be impermeable to rain. Though actually, in shops people usually say waterproof, which is the everyday cousin. Impermeable is the more careful school-and-lab word, and it is not only about rain: it can be about gas as well as water, or about a membrane that nothing should cross.