The ratio of the difference between the viscosities of solution and solvent to the viscosity of the solvent, i.e. ${\eta }_i=\frac{\eta -{\eta }_{\text{s}}}{{\eta }_{\text{s}}}$, where $\eta$ is the viscosity of the solution and ${\eta }_{\text{s}}$ is the viscosity of the solvent. The use of the term 'specific viscosity' for this quantity is discouraged, since the relative viscosity increment does not have the attributes of a specific quantity.