What property of a fluid describes its ability to resist flow?

Viscosity is the property of a fluid that describes its ability to resist flow. It essentially measures the internal friction within the fluid, which affects how easily it can deform and move. A fluid with high viscosity, such as honey, flows slowly because its internal structure impedes movement. Conversely, a fluid with low viscosity, like water, flows more freely as there is less internal resistance to flow.

Temperature can influence viscosity; for example, increasing temperature typically reduces viscosity in liquids, making them flow more easily. Density is a measure of mass per unit volume and does not directly indicate how a fluid will behave in terms of flow. Pressure, while it can affect fluid movement, particularly in compressible fluids, does not define the fluid's inherent resistance to flow but rather how flow is influenced under varying conditions. Thus, viscosity is the crucial characteristic that directly relates to a fluid's resistance to flow.