1. (hydraulic conductivity) In general, the ability of a rock, sediment, or soil to permit fluids to flow through it. More precisely, the hydraulic conductivity is the volume flow rate of water through a unit cross-sectional area of a porous medium under the influence of a hydraulic gradient of unity, at a specified temperature. It is measured in units of m/s or m/day and varies with temperature. Typical values range from 10−6 m/day for clay to 103 m/day for coarse gravel. The magnitude of hydraulic conductivity depends on the properties of both the fluid and the medium. An alternative measure, used in the oil industry, is intrinsic permeability, measured in m2 (or in industrial units called darcies), which depends on the properties of the rock alone.
2. Property of a membrane or other barrier, being the ease with which a substance will diffuse or pass across it.
3. Capacity of a material to transmit fluids, expressed as hydraulic conductivity. In soils, the ease with which gases, liquids, or plant roots penetrate into or pass through a layer of soil.
4. (magnetic permeability) The ratio of the magnetic flux density in a medium to the magnetizing force. In free space (i.e. air) this is a constant, μ0 equal to 4π × 10−7.