A Newtonian fluid is a type of fluid that maintains a constant viscosity regardless of the applied stress or shear rate. This means that when you stir or move a Newtonian fluid, its resistance to flow remains the same, making it predictable and easy to work with. Common examples include water, air, and most oils. In contrast to non-Newtonian fluids, which change their viscosity under stress, Newtonian fluids behave consistently. This property is essential in various applications, such as engineering and fluid dynamics, where understanding fluid behavior is crucial for designing systems and processes.