What term describes the internal resistance of a fluid that impedes flow?

Viscosity is the term that describes the internal resistance of a fluid that impedes flow. It measures a fluid's thickness or stickiness, indicating how easily it can move under applied forces. Higher viscosity means greater resistance to flow, while lower viscosity allows a fluid to flow more freely. This property is crucial in hydraulics and pneumatics, as it directly affects how fluids behave when being pumped through systems, determining factors like the speed of flow and the efficiency of energy transfer. Understanding viscosity is vital for engineers and technicians when selecting fluids for hydraulic and pneumatic applications, ensuring optimal system performance.

The other terms provided do not capture this specific trait of fluids. Pressure refers to the force per unit area exerted by a fluid, density relates to mass per unit volume, and viscidness is often used interchangeably with viscosity but is less common in technical discussions. Therefore, viscosity is the most appropriate and precise term for describing the resistance to flow within a fluid.