Mean free path
The mean free path is an important concept in physics; it is the average distance travelled by a particle in a material before colliding with another particle in the material. It is inversely proportional to the density of particles in the material.
It was Initially introduced in the kinetic theory of gases and it is also used in particle physics to define the penetrating power of a beam of particles through a material.
A molecule travels on average 70 nm through air before colliding with another molecule.