In fluid testing, viscosity refers to what?

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Viscosity is a fluid's resistance to flow. It describes how strongly the molecules in a liquid stick to each other and to surfaces, creating internal friction that makes movement harder. That’s why water pours easily while honey resists and flows slowly. Temperature affects viscosity: heating lowers it by giving molecules more freedom, while cooling raises it as movement slows. In practical terms, viscosity determines how much pumping force is needed and how quickly a fluid will move through pipes or channels. It’s measured with a viscometer and given in units such as pascal-seconds or centipoise. This property is different from density (mass per volume), color, or refractive index (how light bends in the fluid).