The term for the mass of a substance per volume is called density. Density is a crucial physical property that indicates how much matter is packed into a given space. It allows for comparisons between different materials and can influence behaviors in various applications, such as buoyancy in fluids and thermal conductivity in solids. Density is mathematically expressed as mass divided by volume (density = mass/volume), and its unit of measurement typically includes grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³).