Viscosity is the measure of a fluid's resistance to flow. In simpler terms, it refers to the thickness or stickiness of a fluid. Viscosity is an important property of fluids as it affects the rate at which they flow, their ability to lubricate, and their resistance to deformation. High-viscosity fluids are thicker and flow more slowly, while low-viscosity fluids are thinner and flow more quickly. Viscosity is commonly measured in units called poise or centipoise.