In thermodynamics, a body is a term referring to any volumetric region of matter or space (containing matter) that can be acted on by heat in a determinate manner. [1] The term varies in use according to context, e.g. a "hot body" (boiler) or "cold body" (condenser), but is generally used in reference to the quantity of matter made to do work via contraction or expansion, according to Boerhaave's law, being synonymous with the terms working substance (1824), working body (1850), system (1871), thermodynamic system (1923), working medium (2007), among others. [1]

Etymology
The “body”, in a mechanical theory sense, was first defined in 1783 by French engineer Lazare Carnot as such: [2]

“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.”

In 1824, building on the work of his father, French physicist Sadi Carnot, in laying out the framework of the soon-to-be established science of thermodynamics, stated that in order to understand “the phenomenon of the production of motion by heat,” whenever heat acts in a determinate manner “on any body”, that the laws of physics would need to be extended and generalized enough to make known beforehand all of the effects that occur in reference to the relations between heat and motion. [3]

In setting up a structural framework for his analysis, Carnot conceived of a generalized heat engine to which the fluid or malleable “body” (in general reference to a body of steam) moving about inside of a steam or heat engine, he referred to as the “working substance”. This body could be made to do work, via expansion or contraction, by either receiving caloric (particles) from contact with boiler (or furnace), which he referred to as the “warm body” or by rejecting caloric (particles) via contact with the condenser (or stream of cooling water), which he referred to as the “cold body”.

References
1. (a) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (I)”, (pgs. 257-59). Nature, Jan. 31.
(b) Maxwell, James C. (1878). “Tait’s ‘Thermodynamics’ (II)”, (pgs. 278-81). Nature, Feb. 07.
2. Lazare, Carnot. (1783). Essay on Machines in General, art. 8, trans. Ivor Grattan-Guinness.
3. Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power.” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.