The average distance travelled between collisions by the molecules in a gas, the electrons in a metallic crystal, the neutrons in a moderator, etc. According to the kinetic theory the mean free path between elastic collisions of gas molecules of diameter d (assuming they are rigid spheres) is 1/√2nπd2, where n is the number of molecules per unit volume in the gas. As n is proportional to the pressure of the gas, the mean free path is inversely proportional to the pressure.