What does viscosity measure in a fluid?

Viscosity is a measure of a fluid's resistance to flow, often described as the "thickness" or "stickiness" of the fluid. When a fluid has a high viscosity, it flows more slowly and requires more force to move, which is characteristic of thicker liquids like honey or molasses. Conversely, fluids with low viscosity, such as water or alcohol, flow easily and quickly.

In practical applications, understanding viscosity is essential in fields such as medicine and engineering, particularly when considering how blood flows through vessels, how lubricants behave, or how different fluids interact in various systems. Recognizing viscosity helps predict how a fluid will behave under different conditions, ultimately aiding in fluid dynamics assessments and related diagnostics. The other options refer to different properties; density relates to mass per unit volume, temperature measures thermal energy, and volume is the space occupied by the fluid, which do not directly address the concept of how resistance to flow relates to thickness.