In thermodynamics, as system, based on the heat engine, is differentiated into three types of bodies: cold body, i.e. a low temperature body, such as cold water, hot body, i.e. a high temperature body, such as boiling water, and a working body, i.e. the substance, confined within a volume, such as water within a piston and cylinder, which is generally thought of as the system, that can be made to expand or contract, via contact with the hot body and cold body, respectively.
The following are quotes:
- “When a body acts upon another on, it is always immediately or through some intermediate body; this intermediate body is in general what one calls a ‘machine’.”
- — Lazare Carnot (1783), Essay on Machines in General (pg. #) 
- Lazare, Carnot. (1783). Essay on Machines in General (Translator: Ivor Grattan-Guinness) (art. 8). Publisher.
- Body – Hmolpedia 2020.