A term used mainly in liquid chromatography. It compares the mobile phase velocity with the velocity of diffusion into the pores of the particles (the so-called diffusion velocity, u D): \[\nu=\frac{\overline{u}}{u_{\text{D}}}=\frac{\overline{u}\ d_{\text{p}}}{D_{\text{M}}}\] In open-tubular chromatography: \[\nu=\frac{\overline{u}\ d_{\text{c}}}{D_{\text{M}}}\]