Energy is taken to mean that which is necessary for a living system to continue its activity and to implement activity decisions

from Wikipedia

Energy is so fundamental that it is not easily defined in terms of anything more fundamental.

For a general audience, rather than worrying about the details of a formal definition, it is far easier and far more useful to understand what energy does in various situations.


This is called the "energy is as energy does" school of thought. This approach emphasizes examples of energy, the strict local conservation of energy, and the connection between energy and other quantities of interest. Thus, some textbooks1 take the view that energy is primary and fundamental, and introduce it as a thing in itself, without relying on prior notions of force or work. Other textbooks first discuss work, and then introduce energy via equation (1) (below). That is appropriate for introductory purposes, but should not be carried too far.