Density is a physical property of a substance that relates its mass to its volume. It is defined as the amount of mass per unit volume of a substance, usually expressed in grams per cubic centimeter (g/cm3) or kilograms per cubic meter (kg/m3).

In other words, density is the measure of how tightly packed the molecules or particles of a substance are. The denser a substance is, the more tightly packed its molecules or particles are, resulting in a higher mass per unit volume.

The density of a substance can be affected by factors such as temperature and pressure. For example, as the temperature of a substance increases, its density usually decreases, as the particles move more and take up more space, resulting in a lower mass per unit volume. Similarly, as the pressure on a substance increases, its density usually increases, as the particles are forced closer together, resulting in a higher mass per unit volume.

The density of a substance is an important physical property, as it can be used to identify a substance, determine its purity, and even predict its behavior in chemical reactions.