A Knudsen gas is a gas in a state of such low density that the mean free path of molecules is greater than the diameter of receptacle that contains it. The molecular dynamical regime is then dominated by the collisions of the gas molecules with the walls of the receptacle rather than with each other.
A flow of gas through a pipe is called Knudsen flow when it can be expressed as the difference of two independent flows, which enter the pipe through its different ends.
References
Knudsen gas Wikipedia(Text) CC BY-SA