A Newtonian fluid is any fluid that exhibits a viscosity that remains constant regardless of any external stress that is placed upon it, such as mixing or a sudden application of force. An example of a Newtonian fluid is water, since it flows the same way regardless of whether it is left alone or agitated vigorously. This can be contrasted with non-Newtonian fluids, which can become thicker or thinner when stress is applied. It is possible for the viscosity of a Newtonian fluid to change if it is exposed to different temperatures or pressures instead of external applications of force. Many Newtonian fluids become thicker as they are cooled, though they still react to shear forces without a change in viscosity.

The defining factor of any Newtonian fluid is that it will flow the same when a great deal of force is applied as when it is left alone. This means that Newtonian fluids can be mixed vigorously without changing in viscosity. Another way to describe these fluids is that they have a linear relationship between viscosity and shear stress. Regardless of the shear stress applied to these fluids, the coefficient of viscosity will not change.

Many different fluids are Newtonian, including water, certain oils, and air. The importance of this fact can be seen when examining many different non-Newtonian fluids. Some fluids exhibit shear thickening, which means that the viscosity increases in the presence of an external force. If water or air exhibited these properties, runners and swimmers would find the air and water thickening around them as they attempted to move faster. This type of effect can be seen when water and corn starch are mixed to create a non-Newtonian fluid, which is liquid if left alone but will solidify and allow someone to run across it.

It is possible for the viscosity of a Newtonian fluid to change through factors other than externally applied force. Temperature and pressure are two methods that can be used to change the viscosity of these fluids. Compressible liquids will tend to become thicker under pressure, while incompressible liquids exhibit a negligible change under the same circumstances. Newtonian fluids can also change in density when exposed to extreme temperatures, which can lead to an increase or decrease in viscosity. A fluid that has changed viscosity through one of these methods will still show a linear relationship between viscosity and shear stress.