noun: a usually fluid substance that through changes of temperature, volume, and pressure is the means of carrying out thermodynamic processes or cycles (as in a heat engine) Merriam-Webster, Working Substance
In 1824 Sadi Carnot described a thermodynamic system as the working substance (such as the volume of steam) of any heat engine under study. The very existence of such thermodynamic systems may be considered a fundamental postulate of equilibrium thermodynamics, though it is only rarely cited as a numbered law. According to Bailyn, the commonly rehearsed statement of the zeroth law of thermodynamics is a consequence of this fundamental postulate. Wikipedia, Working Substance
See Also
Boyles Law
Carnot Cycle
Carnot Theorem
Laws of Thermodynamics
Russells Laws of Thermodynamics
Thermodynamics
through-flowing substance
working fluid