permeability [geol] (per''-me-a-bil'-i-ty). The property or capacity of a porous rock, sediment, or soil for transmitting a fluid; it is a measure of the relative ease of fluid flow under unequal pressure and is a function only of the medium. The customary unit of measurement is the millidarcy. Cf: absolute permeability; effective permeability; relative permeability; hydraulic conductivity. Syn: perviousness. Adj: permeable. Ant: impermeability.