A measure of a fluids resistance to flow. A higher viscosity is essentially a thicker fluid. a fluids viscosity is its VI (Viscosity Index) or how a fluids viscosity is affected by temperature. It’s a measure of the rate of change of viscosity with temperature. In lubricants a high VI is desirable to maintain a relatively consistent viscosity throughout the useable temperature range. In heat transfer fluids a lower VI is more desirable in order to allow the fluid to thin out ( reduce viscosity ) with temperature increases. This allows for more efficient thermal transfer properties