Physical quantity with the special property that, in physical processes, energy is neither destroyed nor created, simply transformed from one form of energy to another. Some of the different forms of energy that are defined separately are kinetic energy, thermal energy and the energy carried by electromagnetic radiation.
Processes that transform one form of energy into another take place in all machines we use in everyday life, from the engine of a subway train (electrical energy into kinetic energy of the train) to an electric blanket (electrical energy into thermal energy).
One important consequence of special relativity is that energy and mass are completely equivalent – two different ways to define what is, on closer inspection, one and the same physical quantity. See the keyword equivalence between mass and energy.