👉 The textbook fluid, often denoted as "viscosity," is a fundamental property of fluids that describes their resistance to flow. It quantifies how easily a fluid can move or deform under an applied force; higher viscosity means the fluid is thicker and flows more slowly, while lower viscosity indicates a thinner, more fluid substance that moves more readily. In textbooks, this concept is crucial for understanding various fluid dynamics phenomena, from the flow of blood in arteries to the behavior of oil and water. It is typically measured in units like Pascal-seconds (Pa·s) or centipoise (cP), and its behavior is governed by principles such as Newton's law of viscosity, which links shear stress to the rate of deformation in the fluid.