What is the term for a fluid's tendency to resist changes in its velocity?

The term that best describes a fluid's tendency to resist changes in its velocity is viscosity. Viscosity refers specifically to the internal friction within a fluid, which determines how easily it flows. A fluid with high viscosity, like honey, resists motion more than a fluid with low viscosity, like water. Therefore, when considering how fluids behave under motion and the force required to change that motion, viscosity is the key concept.

Inertia, while related to motion, specifically refers to an object's resistance to change in its state of motion. It is a property of mass and does not directly pertain to the characteristics of fluid motion itself.

Fluid dynamics is the broader study of fluids in motion, encompassing all aspects of fluid flow, but does not define the specific property of resisting changes in velocity.

Momentum involves the quantity of motion an object possesses, which depends on both mass and velocity. While both viscosity and momentum are integral to the study of fluids, only viscosity specifically addresses the resistance to flow and changes in velocity.