Viscosity of a hydraulic fluid at a defined temperature. This used to be +50°C according to DIN 51 502 and has been +40°C for a number of years now according to ISO 3448 (DIN 51 519). It is essential therefore when thinking about nominal viscosity to consider the reference temperature used.
Nominal viscosity represents an average value within the respective viscosity class (e.g. ISO viscosity class 32 ranges from around 29 mm2/s to 35 mm2/s).