Newtonian flow
A Newtonian fluid is the simplest, best-behaved kind of liquid: its viscosity is a fixed number that does not change no matter how hard or how fast you push it. Water, simple syrups, glycerin, and mineral oil are everyday Newtonian examples. Stir them gently or violently and they feel equally 'thick' relative to the effort applied.
On a rheogram (shear stress plotted against shear rate), a Newtonian fluid gives a straight line passing through the origin. The slope of that line is the viscosity, and because the line is straight, that single slope value fully describes the fluid. This is why a single viscosity figure is meaningful for these liquids but not for more complex ones.
Most simple single-phase pharmaceutical solutions behave in a Newtonian manner, which makes them easy to specify and measure. The complications begin once you add particles, polymers, or two phases, which is why creams, gels, and suspensions usually depart from Newtonian behaviour and require fuller rheological characterisation.
Simple syrup, a concentrated sucrose solution used as a sweet vehicle, flows in a Newtonian way, so its quality can be controlled by a single specified viscosity value.
For Newtonian liquids one viscosity number tells the whole story.
Newtonian flow is named after Isaac Newton, who first proposed the proportionality between shear stress and shear rate; it is the reference case against which all 'non-Newtonian' fluids are defined.