porous

The word "porous" describes a material that contains pores, which are tiny openings or spaces within its structure. These openings let fluids such as liquids or gases pass through or collect inside, depending on the material. You can think of it as a solid that has a hidden network of very small gaps.

While we often picture sponges first, many materials can be porous to different degrees. Pumice stone, used for smoothing skin, is very porous, which is why it feels light and rough. Wood can also be porous and may absorb water if left untreated. Even materials that seem dense, such as limestone or sandstone, can have tiny spaces that let water slowly seep through over time.

The degree of porosity affects how a material behaves. Highly porous materials often absorb liquids quickly, like a sponge, but they may also be weaker. In geology, the porosity of rock matters because it helps store groundwater, oil, and gas. In the body, some structures are porous enough to allow the movement of fluids or the growth of tiny blood vessels.

"Porous" can also be used metaphorically. You might say a border is porous if people can cross it too easily, or that a security system is porous if it is full of weak spots. In these cases, the idea is the same: something that should block entry has too many openings.

So whether you are talking about the physical structure of a material or the weak points in a system, "porous" suggests the presence of openings that let something pass through. Depending on the context, that can be useful, natural, or a problem.