A measure of the continuum assumption of fluids; that fluids behave as a continuous medium, or continuum:
$
Kn=\frac{\lambda}{L}=\frac{\text{Molecular meen free path}}{\text{Characteristic length}}
$
$
\begin{align}
Kn&\ll1\quad\text{Continuum assumption closely obyed} \\
Kn&\gg1\quad\text{Free molecule flow (rarefied gas flow)}
\end{align}
$