Answers (1) Jewels Vern. Energy is sometimes defined as capacity to do work. But work is defined as disordering energy. So energy is capacity to do disordering energy. That is a circular definition. Energy is not a physical unit. It is a philosophical concept and an accounting technique used to analyze mechanical and chemical exchanges. In nuclear physics it is defined as a wavelength of light, in biology it is a synonym for metabolism, and in public utilities they say energy when they mean connectivity. Anybody who uses the word in any other context does not know what he is talking about.