What is mean free path?

(N/A) The mean free path is defined as the average distance traveled by a gas molecule between two successive collisions.
If a molecule travels distances $\lambda_1, \lambda_2, \lambda_3, ..., \lambda_n$ between $n$ successive collisions,the mean free path $\lambda$ is given by:
$\lambda = \frac{\lambda_1 + \lambda_2 + ... + \lambda_n}{n}$.
According to the kinetic theory of gases,it is expressed as $\lambda = \frac{1}{\sqrt{2} \pi d^2 n}$,where $d$ is the diameter of the molecule and $n$ is the number density of the molecules.