These words refer to the resistance of a thing to incursions upon or into it. Impenetrable suggests a solid mass of resistance that cannot be pierced or breached: miners trapped in the tunnel by an impenetrable mass of debris; rush-hour traffic that is almost impenetrable to the poor pedestrian; an impenetrable phalanx of screaming women clustered round the bargain table. The suggestion of an unbroken mass, however, need not always be present; in this case the word may simply suggest the considerable resistance presented to anyone attempting to move through the medium: an impenetrable clump of blackberries. The word may even be applied to things that permit incursions but thwart their purpose: a jungle that was impenetrable to a whole generation of explorers, all of whom it swallowed up with effortless ease. Used of people, their expressions or personality, the word suggests inscrutability, lack of response, or hostility: She responded to his proposal with an impenetrable blankness. The failure to pierce an impenetrable object might still leave marks or superficial signs of damage. By contrast, impervious suggests something that is literally beyond showing any change whatsoever from attempts to breach or affect it: gems that are impervious to scratching or to wear; metals impervious to cold and heat. As can be seen, this word does not necessarily imply warding off efforts at piercing something so much as surviving harsh vicissitudes without change. This is even more true in the word’s less concrete uses, suggesting inscrutability or hostility less than inner strength, stubbornness or determination that cannot be swayed by externals: They led a life impervious to criticism; infuriatingly impervious to her every suggestion; impervious to the temptations offered him if only he would compromise his position.

Impassable and impermeable are both more narrowly restricted to specific contexts of resistance than the foregoing. Impassable refers to any blockage that makes travel impossible: roads that became impassable during the rainy season; heavy snows and avalanches that rendered the mountain route impassable . Impermeable refers, even more specifically, to a membrane through which certain fluids cannot pass. The membrane itself, of course, is not the same sort of barrier to passage as those implied by these other words, since it might well be easily ruptured; also, the membrane may be selective in the substances to which it prohibits passage: tissues impermeable to carbon dioxide but not to oxygen.