A term used mainly in liquid @C01075@. It compares the mobile phase velocity with the velocity of @D01716@ into the pores of the particles (the so-called @D01716@ velocity, \(u_{\text{D}}\)): \[\nu=\frac{\overline{u}}{u_{\text{D}}}=\frac{\overline{u}\ d_{\text{p}}}{D_{\text{M}}}\] In open-tubular @C01075@: \[\nu=\frac{\overline{u}\ d_{\text{c}}}{D_{\text{M}}}\]