Molecules in a fluid constantly collide off each other. The mean free time
of a molecule in a fluid
is the average
time between collisions. The mean free path
of the molecule is the product of the average speed
and the mean free time. These concepts are used in the kinetic theory of gases
to compute transport coefficients such as the viscosity
In a gas the mean free path may be much larger than the average distance between molecules. In a liquid these two lengths may be very similar.