relative viscosity

Synonym: viscosity ratio
https://doi.org/10.1351/goldbook.R05282
The ratio of the viscosity of the solution, η, to the viscosity of the solvent, η s, i.e. η r = η η s. Synonymous with viscosity ratio.
Source:
Purple Book, 1st ed., p. 62 (http://old.iupac.org/publications/books/author/metanomski.html)