What is the term for a fluid's resistance to flow?

The term for a fluid’s resistance to flow is viscosity. It describes how strongly neighboring layers of a fluid resist sliding past one another, or more precisely, how much the fluid resists deformation at a given rate.

At the molecular level, viscosity comes from different effects in liquids and gases. Cohesive forces between molecules are important in liquids, while molecular collisions dominate much of the resistance in gases. Honey and syrup have higher viscosities than water, which is why they flow more slowly under similar conditions. Temperature also matters: most liquids become less viscous when heated, while gases generally become more viscous.

Viscosity is not the same as density. Density measures mass per volume; viscosity measures resistance to flow. Surface tension acts at a liquid’s surface, and elasticity describes how some materials resist deformation and recover their shape. The SI unit of dynamic viscosity is the pascal-second (Pa·s). In a pipe, viscous flow is usually fastest near the center and slowest next to the walls.