In the context of fluid systems, what is viscosity?

Viscosity is defined as the resistance of a fluid to flow. It describes how thick or thin a fluid is, which directly impacts its ability to move. For instance, honey has a high viscosity compared to water, meaning it flows more slowly and resists movement more than a less viscous fluid like water. Understanding viscosity is crucial in fluid systems, as it affects everything from how fluids are transported through pipes to the performance of pumps and other components.

The other options provided, while related to fluids, do not accurately define viscosity. Density refers to the mass per unit volume of a fluid, which is a separate property. The velocity of liquid flow describes how fast the fluid is moving, not its internal resistance to flow. Pressure, meanwhile, refers to the force exerted by a fluid per unit area, also distinct from the concept of viscosity. Hence, the correct answer highlights the fundamental property of resistance that characterizes fluid motion.