Newtonian fluids are liquids or gases that have a constant viscosity, meaning their flow behavior remains consistent regardless of the applied stress. This means that when you stir or push a Newtonian fluid, it flows smoothly and predictably. Common examples include water, air, and most oils. The term comes from Sir Isaac Newton, who described the relationship between shear stress and shear rate in these fluids. In contrast, non-Newtonian fluids exhibit varying viscosity depending on the stress applied, leading to more complex flow behaviors. Understanding Newtonian fluids is essential in fields like engineering and fluid dynamics.