Molecules in a fluid constantly collide with each other. The mean free time for 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.^[1] These concepts are used in the kinetic theory of gases to compute transport coefficients such as the viscosity.

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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.

Scattering is a random process. It is often modeled as a Poisson process, in which the probability of a collision in a small time interval ${\displaystyle dt}$ is ${\displaystyle dt/\tau }$. For a Poisson process like this, the average time since the last collision, the average time until the next collision and the average time between collisions are all equal to ${\displaystyle \tau }$.^[1]