Energy density is a useful measurement when dealing with electric and magnetic fields. The idea is also used in nutritional research, though the calculation in this discipline is known as caloric density. The value is a useful parameter especially when comparing different fuels. For example, hydrogen fuel has lower energy density than gasoline. A higher energy density indicates that more energy can be stored or transported for the same amount of mass.
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored. Quantified energy is energy that has some sort of, as the name suggests, quantified magnitude with related units.
Energy per unit volume has the same physical units as pressure, and in many circumstances is an exact synonym: for example, the energy density of the magnetic field may be expressed as (and behaves as) a physical pressure, and the energy required to compress a compressed gas a little more may be determined by multiplying the difference between the gas pressure and the pressure outside by the change in volume.