The term density appears in astronomy in many different contexts.

• In its most generic use the density is the mass per unit volume of an object or region and might have units like kg/m³ or Mo/pc³.

• The Number density is the number of a particular object or species per unit volume and might be used when describing the number of electrons per cubic centimetre in a plasma or the number of stars in the core of a globular cluster.

• Density profile is the density as a function of some variable like distance from the centre of a star.

• The mean or average density is the total mass of a region divided by the total volume of that region.

Since the behaviour of matter is often a function of density it is a very important quantity to determine when trying to explain the underlying physics of an object.