Mass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much denser at its center than near its surface. Density is commonly expressed as g/cm3, but the correct SI unit for density is kg/m3 (1 g/cm3 = 1000 kg/m3).