working fluid

The working fluid in a machine is the pressurized gas or liquid which actuates the machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder. More generally, a thermodynamic system consists of a system boundary across which energy transfers occur. The working fluid is that matter contained within the system boundary (which can be a liquid or gas) which absorbs or transmits these energies.