In kinetic theory of gases, the mean free path is the average distance traveled by a molecule, or a number of molecules per volume, before they make their first collision. Let  be the collision cross section of one molecule colliding with another. As in the previous section, the number density  is defined as the number of molecules per (extensive) volume, or  . The collision cross section per volume or collision cross section density is , and it is related to the mean free path  by
Notice that the unit of the collision cross section per volume  is reciprocal of length.