relative viscosity increment

The ratio of the difference between the viscosities of solution and solvent to the viscosity of the solvent, i.e. $η i = η − η s η s$ , where $η$ is the viscosity of the solution and $η s$ 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.

Source:

Purple Book, p. 63