We have many different ways of describing and quantifying 'energy' eg kinetic, potential etc. We also know that mass and energy are equivalent. We intuitively know what it does and we talk about it across many fora but my question is 'What is Energy?'. What is this characteristic of the universe whose effects we freely discuss. Is it possible to define Energy in a non-tautological manner ie without reference to matter or forms and manifestations of energy?