"What is energy? Pick up a chemistry text, a physics text, or a thermodynamics text, and look in the index for 'Energy, definition of,' and you find no such entry."

"Everytime they have an opportunity to to define energy, they fail to do so. Why the big secret? Or is it presumed you already know? Or is it just obvious?"

"Energy can be defined as the capacity to supply heat or do work."

Richard Feynman, Six Easy Pieces (excerpts from the Feynman Lectures on Physics)

"There is a fact, or if you wish, a law, governing natural phenomena that are known to date. There is no known exception to this law -- it is exact so far we know. The law is called conservation of energy; it states that there is a certain quantity, which we call energy that does not change in manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number, and when we finish watching nature go through her tricks and calculate the number again, it is the same."

"It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount."

Paraphrasing Feynman, there is something quantitative that is conserved in the universe, and we call it energy.