The ratio of shear stress to shear rate.
It is a fluid's internal resistance to flow.

The common unit of absolute viscosity is Poise and CentiPoise(cP).

Absolute viscosity divided by the fluid's density equals kinematic viscosity.

Absolute viscosity is typically measured by a rotary viscometers to determine the torque on rotating spindle and so measure the fluid's shear resistance. Changing the rotor(spindle) dimensions and the gap between the rotor and stator wall(container) and the speed of rotation can change the rate of shear.

Instances of rotary viscometers that are used for absolute viscosity measurements:
Cold-cranking Simulator(CCS)
Mini-rotary Viscometer(MRV)
Brookfield Viscometer
Tapered Bearing Simulator

In relation to oils for automative applications such as motor oil or gear oil, the CCS and MRV test equipment at low temperatures is used to determine if the test lubricant does not get too thick to prevent safe engine or transmission operation at low temperatures.

If motor oil is too viscous to flow, even if engine can be started, certain mechanical damage will result due to localized oil starvation. In transmissions both manual and automatic, proper shifting may be impaired, affecting safe vehicle operation once vehicle is put in motion.