In thermodynamics, a thermodynamic system is a synonym, introduced in 1923, of the older terms working substance (1824), working body (1850), or system (1871), and of the newer terms, such as working medium (2007). The term seems to have arisen out of the unwritten practice of placing the clarifier “thermodynamics” in front of whatever term was being used, such as thermodynamic function, thermodynamic arrow, thermodynamic evolution, thermodynamic temperature, etc., in unintentional efforts to help distinguish the term in some sense or degree. In a modern use, however, when diagrams are made, such as shown below, in which the center region is titled thermodynamic system: [2]


History
The term thermodynamic system came of use in the popular the 1923 textbook Thermodynamics by American physical chemists Gilbert Lewis and Merle Randall. [1] They state that what is customarily called a “system”, which can be enclosed either by physical walls or imaginary mathematical surfaces, is whatever part of the objective world that is the subject of thermodynamic discourse, e.g. a crystal or a cubic centimeter portion of crystal. On this definition that surmise:

“A thermodynamic system may contain no substance at all, in the ordinary sense, and consist of radiant energy, or an electric or magnetic field … [but] usually comprises a substance, which may be homogeneous or heterogeneous.”

Difficulties on terminology
The new student of thermodynamics, when introduced to the the term "system" or "thermodynamic system" (such as diagrammed above) may easily fall into the assumption that the “thermodynamic system” is a two-component system, comprising both the system and the surroundings, or a three-component system, comprising the boiler (hot body) A, condenser (cold body) B, and substance (working body):

This thermodynamic "system" model, in turn, stems from the 1850 terminology of German physicist Rudolf Clausius, who referred to the "working body" as the system:
In this sense, where the term "thermodynamic system", as given to the newby thermodynamics student, has little visual connection to the 1824 working substance “steam body” description of heat engine operation by French physicist Sadi Carnot, as depicted below, in which the "system" is the body of water molecule contained in the piston volume region acdb:


This diagram model by Carnot, in turn, was loosely based on the steam engine designed by Scottish engineer James Watt, shown below, having a separate condenser (1765), sun and planet gear (1781), centrifugal governor (1788), where the "system", "working substance", or "working body" is the "body" of an amount of water, heated in a boiler (hot body, A) and fed as steam, through conduit d, into the piston C, later to be condensed into water again via contact with the condenser (cold body, B), diagrammed below as section D, is the steam engine:

All steam engines, Watt's design included, in turn, were modeled on the diagram and design of a "heated" piston-and-cylinder, shown below, configured with the ability to do work, via certain steps of operation, as detailed in the 1690 memoir "A New Method to Obtain Very Great Motive Powers at Small Cost" by French physicist Denis Papin: [4]


Papin's heat engine design, which generally contains the first verbal description of what later came to be called the "Carnot cycle", in turn, was based on the very-popularized 1647 piston-and-cylinder vacuum pump weight-lifting ideas initiated in 1647 by German inventor and physicist Otto Guericke, such as the one shown below:

In a experiment shown to the left, 20+ men pulled on the piston till it reached halfway up position, then a large glass receiver, which had been mad perfectly vacuous by Guericke’s pump, was then applied to the stop-****; and when the men were exerting their utmost force, a communication was opened between the receiver and the cylinder, and the piston was suddenly forced down to the bottom of the cylinder in spite of the efforts of the men to keep it up. In the experiment shown to the right, conducted in 1654, Guericke arranged the piston at the top of the cylinder, having a scale loaded with 2,686 lbs attached to it. In this configuration, a little boy, by means of a small syringe applied at the stop-**** x to pump out the air, was able to bring down the piston and raise the weight. [2]

In sum, these early piston and cylinder experiments, in later connective coordination, via the caloric theory, to Dutch physician and chemist Herman Boerhaave’s 1724 heat augmentation axiom, are to which the conception of a thermodynamic "system" stem. In this sense, the modern joint term "thermodynamic system" may not be an advisable one, and may lead to confusion, particularly in regards to the “theory of transformation-equivalents”, introduced in the period 1850-65 by German physicist Rudolf Clausius, otherwise known as entropy. [3]

A thermodynamic system view of a surface section of the earth.

Earth-bound thermodynamic systems

See main: Earth-bound thermodynamic systems

In subjects such as life thermodynamics, evolutionary thermodynamics, biological thermodynamics, ecological thermodynamics, economic thermodynamics, sociological thermodynamics, or human thermodynamics in general, the diagramming of the boundary of the thermodynamic system becomes a paramount issue. The basic model delineates a cylinder type imaginary volumetric region on the surface (substrate) of the earth, viewed such, while remaining in contact with the surface of the earth, it rotates in a 24-hour, two-part heat cycle, being put in contact with a hot body (the sun) for approximately 12-hours, the expansion phase, and then put in contact with a cold body (the night sky) for approximately 12-hours, the contraction phase, whereby after the body, if it is considered to be reversible, returns to its original condition.

References
1. Lewis, Gilbert N. and Randall, Merle. (1923). Thermodynamics and the Free Energy of Chemical Substances, (pgs. 8-9). New York: McGraw-Hill Book Co., Inc.
2. Zdunkowski, Wilford and Bott, Andreas. (2004). Thermodynamics of the Atmosphere - A Course in Theoretical Meteorology. (pg. 2). Cambridge: Cambridge University Press.
3. (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.
(c) Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies. (Google Books). London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
4. (a) Papin, Denis. (1690). “A New Way to Obtain Very Great Motive Powers at Small Cost” (Nova Methodus ad Vires Motrices Validissimas levi Pretio Comparandas). Acta Eruditorum, anno, Aug., pgs. 410-14.
(b) Muirhead, James. (1859). The Life of James Watt, (English translation: Ch. XI, Denys Pain: His memoir of 1690, Section: A New Way to Obtain Very Great Motive Powers at Small Cost”, pgs. 131-42). London: John Murray.