Sentence:  A measure for when the <WSD>continuum hypothesis</WSD> can be considered appropriate is the Knudsen1 number, Kn, which is defined as the ratio of a characteristic molecular length scale, e.g., the mean free path of molecules, and a characteristic length scale of macroscopic flow properties, e.g. a length scale of the smallest scales of fluid motion. 
Meaning of the word marked with <WSD> tags:  The assumption, for the purposes of mathematical modelling, that the material being studied is a continuous mass rather than being composed of discrete particles.