What is the primary measure of a substance’s flow resistance known as?

The primary measure of a substance’s flow resistance is known as viscosity. Viscosity quantifies how easily a fluid can flow; it describes the internal friction within the fluid that opposes motion. For instance, honey has a higher viscosity than water, meaning it flows more slowly because it resists motion due to its thicker consistency.

Density refers to the mass of a substance per unit volume, which does not directly relate to how it flows. The viscosity index is a dimensionless number that indicates how much a fluid's viscosity changes with temperature, but it is not a direct measure of flow resistance itself. Fluidity, although related, is the reciprocal of viscosity; it describes how freely a fluid can flow, but it does not serve as the primary measure of flow resistance. Thus, viscosity is the most accurate term for describing a fluid's resistance to flow.