Definition of viscosity index
: an arbitrary number assigned as a measure of the constancy of the viscosity of a lubricating oil with change of temperature with higher numbers indicating viscosities that change little with temperature
First Known Use of viscosity index
Learn More about viscosity index
Britannica.com: Encyclopedia article about viscosity index
Seen and Heard
What made you want to look up viscosity index? Please tell us where you read or heard it (including the quote, if possible).