The ratio of the @V06627@ of the solution, \(\eta \), to the @V06627@ of the solvent, \(\eta _{\text{s}}\), i.e. \(\eta _{\text{r}} = \frac{\eta}{\eta _{\text{s}}}\). Synonymous with viscosity ratio.
Source: Purple Book, 1st ed., p. 62 [Terms] [Book]