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.
Purple Book, 1st ed., p. 63 (