★ The flow that is moving over the body i.e. in a circular cylinder of diameter d is the continuum flow, whereas the flow that consists of individual molecules moving in random motion is the free molecule flow.

★ The mean free path (?) defines the mean distance between the collisions of the molecule and if this path (?) is smaller than the scale of the body measured (d) then the flow of the body is considered as continuum flow.

★ The path (?) that is of same order as the body scale then the gas molecules then the body surface will have an impact of the molecules and this is known as free molecular flow.