What does viscosity measure?

Answer 1

Viscosity is the measure of the shear stress vs the shear strain of the fluid (in one tensorial direction). It is a derived property of fluids and cannot be directly linked to fluid parameters.

There are typically two ways to measure fluid viscosity, test it in simple shear flow, and test it in elongation. Both tests will yield a fluids viscosity, although the results may be different.

Definition- the state of being thick, sticky, and semifluid in consistency, due to internal friction.

Let it be noted that 'viscosity' is only a term applied to newtonian fluids, because in fact some fluids do not have constant viscosity. This becomes very apparent when attempting to use the Navier Stokes equations on non-newtonian fluids. The 'viscous' closure term falls apart for these fluids. This is once again due to the fact that when you measure the 'viscosity' of a fluid, you are only measure one direction in the stress tensor, which for these fluids is not equal for all directions (seen in cases such as die swell). *See rheology for more information about this topic

Answer 2

Mass