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 σ σ be the collision cross section of one molecule colliding with another. As in the previous section, the number density n is defined as the number of molecules per (extensive) volume, or n = N / V . The collision cross section per volume or collision cross section density is n σ σ , and it is related to the mean free path l by l = 1 n σ σ 2