Permeability refers to the ability of a substance or material to allow other things, such as fluids like water or gases, to pass through it. It is often used in relation to porous materials that have small openings or spaces within them which can be penetrated by the passage of fluid or gas molecules. The degree of permeability depends on factors such as the size and shape of the openings, the pressure applied, and the nature of the substance being passed through. In general terms, materials with high permeability are more easily penetrable than those with low permeability.