What is the property of a fluid that resists the force tending to cause it to flow?

Viscosity is the property of a fluid that resists the force tending to cause it to flow. It measures a fluid's internal resistance to deformation, which can be thought of as its thickness. For example, honey has a higher viscosity than water, meaning it flows more slowly and resists motion more effectively. This characteristic is essential in various applications, such as lubrication, fluid dynamics, and engineering systems where the movement of liquids is involved.

While density refers to the mass of a fluid per unit volume and pressure relates to the force exerted by the fluid against the walls of its container, neither of these properties directly relates to the ability of a fluid to flow in response to applied forces. Similarly, volume is a measurement of how much space a fluid occupies and doesn't pertain to the fluid's flow resistance. Hence, viscosity is the appropriate term for describing a fluid's resistance to flow.