What is Energy?

Because energy exists in many interconvertible forms, and yet can't be created or destroyed, its measurement may be equivalently "defined" and quantified via its transfer or conversions into various forms that may be found to be convenient or pedagogic or to facilitate accurate measurement; for example by energy transfer in the form of work (as measured via forces and acceleration) or heat (as measured via temperature changes of materials) or into particular forms such as kinetic (as measured via mass and speed) or by its equivalent mass.