What term describes the friction between the molecules of a liquid or gas?

The term that describes the friction between the molecules of a liquid or gas is viscosity. Viscosity is a measure of a fluid's resistance to flow, which is directly related to the internal friction that arises from the interactions between the molecules within the fluid. When a fluid has high viscosity, it flows more slowly because the molecules are more resistant to movement due to this friction.

In practical terms, this concept is essential in aquatic fitness, as it influences how individuals move through water and how different types of aquatic activities can vary based on the viscosity of the water environment. Understanding viscosity helps fitness professionals design effective aquatic workouts and recognize how participants may experience movement resistance differently based on the fluid's characteristics.

Density, fluidity, and pressure are related concepts, but they do not specifically refer to the friction between molecules. Density refers to how much mass is contained in a given volume, fluidity describes the ease with which a fluid flows, and pressure is the force exerted by the fluid per unit area. Each of these terms has its own significance in the study of fluids, but viscosity specifically captures the idea of molecular friction.