Abstract
As for the definition of the traditional thermal efficiency, the handling of the quantity and the condition was not often clarified. Moreover, it was likely to lack consistency. Then, it is judged that the clarifying the quantity and the system of energy conversion is important. The efficiency of energy conversion will be called "the energy conversion efficiency [η_<EN>]" in the future instead of "the thermal efficiency [η_<th>]". And, it is attempted to restructure a definition consistent according to the classification of the system. "The input energy [Е_<IN>]" is the sum of energy supplied regardless of the form and the supply-method. "The output energy [Е_<OUT>]" is energy converted. And, it is proposed that the energy conversion efficiency should be given by the following expression according each system; η_<EN>=Е_<OUT>/Е_<IN>]_<System>.
