Energy density: It is the amount of energy stored in a given system or region of space per unit volume or mass. Often only the useful or extractable energy is measured, which is to say that chemically inaccessible energy such as rest mass energy is ignored.
It has the same physical formula as pressure. Thus technically it may be called as pressure too.
Sometimes people use the word specific gravity instead of density. Which is incorrect. Energy density is the amount of energy stored per unit volume. Whereas specific gravity is the ratio of the given volume of material to the weight of the same volume of water. Density has its own unit. Whereas specific gravity has no units.