Ratio of the 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 Click on Term to Read More of a substance to the density of a standard substance. The standard is usually liquid water for solids and liquids and air for gases. The density of liquid water under typical conditions is ~1000 kg/m3. The density of air at room temperature near the surface of the earth is approximately 1.2 kg/m3. Specific Attractive force between all matter - one of the four fundamental forces. Click on Term to Read More is a unitless quantity.