Viscosity is a measure of a fluid's resistance to gradual deformation by shear stress. It quantifies the internal friction within a fluid, representing the shear stress required to maintain a specified rate of flow between layers within the fluid. This property is crucial for understanding fluid dynamics and is highly dependent on factors such as temperature, pressure, and composition. Fluids are broadly categorized based on their flow behavior; for instance, Newtonian fluids exhibit a constant viscosity regardless of the applied shear stress, while non-Newtonian fluids demonstrate a shear-dependent viscosity, meaning their resistance to flow changes under different stress levels. The study of viscosity is fundamental to fields ranging from chemical engineering and rheology to biological transport processes.