What term describes mass per unit of volume?

The term that describes mass per unit of volume is density. Density is mathematically defined as the mass of an object divided by its volume and is typically expressed in units such as kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³). This concept is crucial in various fields, including aerodynamics, because it helps determine how substances behave under different conditions, such as changes in pressure or temperature. Understanding density is important for calculating buoyancy, stability, and the performance of aircraft, as it affects lift and drag forces. Therefore, density is the correct term that directly relates to the relationship between mass and volume.