What is a Newtonian Liquid

A Newtonian liquid is one in which the viscosity of the liquid is constant regardless of the stress applied.  That is, the viscosity of the liquid is the same if it is left alone or agitated vigorously.

In other words, Newtonian liquids obey Newton's law of viscosity which states that the shear stress is proportional to the rate of strain.

Water is a common example of a Newtonian liquid.  The viscosity of water is same whether it is still or in an agitated state.  In contrast, a solution of water and corn starch will be liquid when still but will become highly viscous when agitated.

Liquids in which the viscosity changes in response to the application of stress are called non-Newtonian liquids.