energy, E

In mechanics the sum of potential energy and kinetic energy. In thermodynamics the internal energy or thermodynamic energy increase, Δ U, is the sum of heat and work brought to the system. Only changes in energy are measurable. For photons
E = h ν
where h is the Planck constant and ν the frequency of radiation. In relativistic physics
E = m c 2
where c is the speed of light and m the mass.
Source:
Green Book, 2nd ed., p. 12