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Permeability (often symbolized by κ or k) in fluid dynamics and geology is the ability of materials to allow fluids to pass through them. The common measure of permeability is defined by Darcy's law: this measure is determined by testing the material and plugging values into the (Darcy's law) equation. This permeability represents the contribution of the type of porous material to the rate at which fluid passes through.
The term permeability is also used in electromagnetism (magnetic permeability, μ), in effect, treating a magnetic field as a flowing fluid and quantifying a material's ability to allow the magnetic field lines to pass through (exist within) the material.