When Feynman wrote, "It is important to realize that in physics today, we have no knowledge of what energy is," he was recognizing that although we have expressions for various forms of energy from kinetic to elastic, we seem to have no idea of what the all-encompassing notion of "energy" "is": This paper addresses that issue offering a definition of energy in terms of the more accessible concept of "change." The various forms of energy ("1/2mv[superscript 2]", "mgh", "1/2kx[superscript 2]", "qV", "mc[delta]T", "1/2I[omega][superscript 2]", "1/2CV[superscript 2]", etc.) are abstractions not directly observable. We see a cannonball rise in the air; we do not see potential energy ("PE"). We see physical change, not energy.