The Knudsen number K is usually defined for a gas as the ratio
where 1 is the mean free path and L is any macroscopic dimension of interest. The importance of this number is that, when K << 1, the gas behaves as a continuum fluid on the length scale L. The regime in which K << 1 is known as the hydrodynamic regime while that in which K >> 1 is known as the Knudsen regime. In the former, the gas obeys the Navier-Stokes equations of hydrodynamics while in the latter, rarefied gas dynamics apply.
Truesdell, C. and Muncaster, R. G. (1980) Fundamentals of Maxwell's Kinetic Theory of a Simple Monatomic Gas. Academic Press. New York.