What term describes the weight per unit volume?

The term that best describes the weight per unit volume is density. Density is a fundamental property of materials, defined as mass divided by volume. It can be understood as the concentration of mass in a given volume, giving insight into how heavy a substance is relative to the space it occupies. In practical terms, understanding density is crucial in aviation and various engineering applications, as it influences buoyancy, stability, and material selection.

While mass density and weight density are commonly used terms, density alone encompasses both mass per unit volume and weight per unit volume, depending on the context. Specific gravity relates the density of a substance to that of water and does not directly quantify weight per unit volume.

The precise wording of the question is key, as "weight per unit volume" translates directly to density in general usage, making it the best choice.