The physical property that determines the density of liquids and gases is mass per unit volume, also known as density itself. Density is defined mathematically as the ratio of mass (the amount of matter in an object) to volume (the amount of space the object occupies). It is typically expressed in grams per cubic centimeter (g/cm³) for solids and liquids or kilograms per cubic meter (kg/m³) for gases. The density of a substance can vary with temperature and pressure, particularly for gases, as they can compress or expand. Understanding density is essential in various scientific fields, including chemistry, physics, and engineering, as it influences buoyancy, pressure, and material selection.