Energy density is the amount of
energy stored in a given system or region of space per unit
volume or
mass, though the latter is more accurately termed
specific energy. Often only the
useful or extractable energy is measured, which is to say that chemically inaccessible energy such as
rest mass energy is ignored. In
cosmological and other
general relativistic contexts, however, the energy densities considered are those that correspond to the elements of the
stress–energy tensor and therefore do include mass energy as well as energy densities associated with the pressures described in the next paragraph.