A Permeability is the degree to which groundwater can move freely through an Aquifer measured by the interconnection of pores and fractures

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Other /More definition:
A permeability is the ability of a material to allow the passage of a liquid, such as water through rocks. Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas unpermeable material, such as clay, don't allow water to flow freely.