∞ generated and posted on 2016.08.19 ∞
Energy that is sufficiently diffuse that it is not usefully captured to do work.
Another way of putting this is that it is what is left over after work has been done, along with potential energy and whatever it is that has been created by doing the work. You don't (necessarily) stoop down to pick up a penny from the ground because it's just not worth the effort: It still has value, just not enough to justify the effort required to realize that value, just as waste heat is still energy, but not worth much to organisms.
Unlike nutrients, energy does not cycle about ecosystems but instead flows through them. It comes in as potential energy or kinetic energy, is captured in a chemical form by organisms known as producers, is then used to do something such as organism growth, and in the process energy is lost, from the system to the rest of the universe. These losses are due to inefficiencies in the conversion of energy from one form to another. The losses take the form of waste heat, which generally has a value limited only to keeping things warm.