(physics) A property, characteristic of the state of a thermodynamic system, the change of which is equal to the heat absorbed minus the work done by the system.

(physics) The energy released by a nuclear reaction; either through nuclear fission or nuclear fusion / Such energy used as a power source.

(physics) Any of the discrete stable energies that a quantum mechanical system (such as the electrons of an atom) can have. / Used other than figuratively or idiomatically: see energy, level.

(chemistry) The net potential energy liberated or absorbed during the course of a chemical reaction

heat under the ground used to heat water and make steam to turn generator turbines and make electricity.

(physics) The energy needed to separate the constituent parts of an atom or nucleus; equivalent to the mass defect.

(chemistry, physics) The energy required to initiate a reaction. For example, the flame from the fuse of a firecracker provides a small initial amount of energy, after which the explosive reaction proceeds by itself, releasing a considerably larger quantity of energy. A small push given to a stable but top-heavy object may cause it to fall over; the potential energy released during the fall was present in the system all along but could not be realized as long as the object was upright and balanced.

(physics) The sum of the potential energy and kinetic energy of a body or system

Energy in the form of electromagnetic radiation emitted from the Sun; especially that part of this energy that is converted into usable thermal or electrical energy by man.

(physics) All energy that could somehow be converted to mechanical work.