Darcy

Darcy is a measure of the permeability of a rock. (The word comes from Henry Darcy, a nineteenth-century French water engineer.)

One darcy (D) equals apermeability such that one millilitre of fluid, having a viscosity of one centipoise, flows in one second under a pressure differential of one atmosphere through a porous material having a cross-sectional area of one square centimetre and a length of one centimetre.
The working unit is the millidarcy (mD), one thousandth of a darcy. Darcy's law relates to the flow of fluids, especially gas, oil and water, in underground rocks.