|"The energy of the universe is constant."|
|(first main principle)|
|"The entropy of the universe tends to a maximum."|
|(second main principle)|
“Thermodynamics is the study of the principles and laws behind the phenomenon of the production of motion by heat, considered from a sufficiently general point of view, applicable to not only steam engines, but to all imaginable heat engines, whatever the working substance and whatever the method by which it is operated.”
This subject definition, to note, is modified in the sense that Carnot did not use the term 'thermodynamics', nor did he specifically assign a name for the subject he invented, but rather gave the above definition in reference to an 'unnamed subject' that was in need of formulation; the concise name of this subject would be assigned by Irish physicist William Thomson, who introduced the shorthand terms: "thermo-dynamic" (1849) and "thermo-dynamics" (1854) or in its German variant "Mechanische Wärmetheorie" (Mechanical Theory of Heat) (1850) by German physicist Rudolf Clausius.  The key phrase in Carnot's definition being 'whatever the working substance', which takes it cues from Boerhaave's law (1720) on the expansion and contractions of bodies due to heat, which refers to any physical body in the universe.
Thermodynamics, in more detail, studies the process relationship between heat and other forms of energy, such as work, electricity (electrical work), light (radiation), and generalized forces (e.g. elongation work), etc., as can be quantified by measurements, e.g. pressure, volume, and temperature.  The cornerstones of thermodynamics are the four laws of thermodynamics, which define the rules of temperature equivalence (zeroth law), energy conservation (first law), entropy tendencies (second law), and conditions for an absence of temperature (third law). 
|A 1931 meeting of the minds "thermodynamics" dinner party photoshowing, from left to right, thermodynamics founders: Walther Nernst, Albert Einstein, and Max Planck, following by Robert Millikan, grinning, noted for his famous 1909 electron charge determining oil drop experiment, at the house of host Max von Laue, noted relativistic thermodynamics pioneer; in the photo collage can be seen: Ludwig Boltzmann and Rudolf Clausius among others.|
See main: History of thermodynamicsIn a nutshell, thermodynamics is the science, developed largely between 1823 and 1882, that united affinity-chemistry (1718), thermo-chemistry (1870s), thermo-electricity (1822), overthrew the caloric theory, vitalism, perpetual motion theory (replacing them with the kinetic theory, mechanical equivalent of heat, and the conservation of energy respectively), and later functioning to seed the quantum revolution (1900).
The foundations of thermodynamics, according to American mathematical physicist Willard Gibbs, began to be laid in 1850. Specifically, according to Gibbs, the first memoir on thermodynamics published in 1850 by German physicist Rudolf Clausius, entitled “On the Motive Power of Heat, and on the Laws which can be Deduced from it for the Theory of Heat”, “marks an epoch in history of physics”. Moreover, according to the 1889 words of Gibbs: 
“If we say, in the words of Maxwell some years ago (1878), that thermodynamics is ‘a science with secure foundations, clear definitions, and distinct boundaries,’ and ask when those foundations were laid, those definitions fixed, and those boundaries traced, there can be but one answer. Certainly not before the publication of that memoir (Clausius, 1850).”
In 1865, Clausius assembled his nine total memoirs on thermodynamics into the book The Mechanical Theory of Heat; and with the December 1875 publication of the second edition, in his own words, “re-model[ed] the papers that they might form a connected whole, and enable the work to become a text-book of the science”, after which time it can be said that thermodynamics had solidified into a new branch of science.
Historical | Timeline
See main: Timeline of thermodynamicsVisit the moving (sideways scrolling) twenty-foot long pictorial timeline of thermodynamics, a portion of which is shown below, for a quick visual history of thermodynamics:
See main: Definitions of thermodynamicsThe following is partial chronological listing of definitions of thermodynamics:
“It is a matter of ordinary observation, that heat, by expanding bodies, is a source of mechanical energy; and conversely, that mechanical energy, being expended either in compressing bodies, or in friction, is a source of heat. The reduction of the laws according to which such phenomena take place, to a physical theory, or connected system of principles, constitutes what is called the science of thermodynamics.”
|William Rankine |
|1880||“Thermodynamics, or the mechanical theory of heat, is that science which treats of the mechanical effects of heat, and of those mechanical processes by which heat is generated.”||Robert Rontgen and Augustus du Bois |
|1998||“Thermodynamics now designates the science of all transformations of matter and energy.”||Pierre Perrot |
General branches of
See main: Branches of thermodynamicsThermodynamics has it roots in affinity chemistry (1718), thermo-chemistry (1770s), thermo-electricity (1822), thus becoming a tree with foundations in the works of Sadi Carnot, William Thomson, Rudolf Clausius (trunk), and William Rankine, soon thereafter sprouting many branches of thermodynamics, e.g. biological thermodynamics (1926), each with many sub-branches, e.g. protein thermodynamics (1960s).
The subject of the application of thermodynamics to explain human existence has deep roots, beginning with the 1809 human reaction affinity (free energy) theories of Johann Goethe. The term "human thermodynamics", as a the thermodynamic study of systems of human molecules was coined in 1952 by English physicist Charles Galton Darwin, which generally can be considered as the initiation point of this branch of science.
|Visuwords word-association definition of thermodynamics, showing key terms: adiabatic, process, cycle, heat, enthalpy, thermostatics, equilibrium, entropy, and physics. |
1. (a) Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies. London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
(b) Clausius, Rudolf. (1879). The Mechanical Theory of Heat (2nd ed.). London: Macmillan & Co.
(c) Gibbs, J. Willard (1876). The Scientific Papers of J. Willard Gibbs - Volume One Thermodynamics. Ox Bow Press.
2. Thermodynamics (definitions) - EoHT
3. Atkins, Peter. (2007). Four Laws - that Drive the Universe. Oxford: Oxford University Press.
4. Gibbs, Willard. (1889). “Rudolf Julius Emanuel Clausius,” Proceedings of the American Academy, new series, vol. XVI, pgs. 458-65. In The Scientific Papers of J. Willard Gibbs (Volume II).
5. Rontgen, Robert and Jay Du Bois, Augustus. (1880). The Principles of Thermodynamics: With Special Application to Hot-Air, Gas and Steam Engines (Thermodynamics: defined, pg. 3; Quote: “The human body is thus comparable to a steam engine”, pg. 91). John Wiley & Sons.
6. Rankine, William. (1859). A Manual of the Steam Engine and Other Prime Movers (chapter III: “Principles of Thermodynamics”, pgs. 299-478). London: Charles Griffin and Co.
7. Perrot, Pierre. (1998). A to Z of Thermodynamics (pg. 301). Oxford: Oxford University Press.
8. Thermodynamics (definition) - Visuwords.com.
9. 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.
10. (a) Note: the inequality of the 1862 version of the second main principle is reversed as compared to the 1875 version; a further reading of the the first and second edition of The Mechanical Theory of Heat, with focus on the function N, is required to understand this switch.
(b) Clausius, Rudolf. (1962). “Sixth Memoir: On the Application of the Theorem of the Equivalence of Transformations to Interior Work”, Communicated to the Naturforschende Gesellschaft of Zurich, Jan. 27th, 1862; published in the Viertaljahrschrift of this Society, vol. vii. P. 48; in Poggendorff’s Annalen, May 1862, vol. cxvi. p. 73; in the Philosophical Magazine, S. 4. vol. xxiv. pp. 81, 201; and in the Journal des Mathematiques of Paris, S. 2. vol. vii. P. 209.
11. Maugin, Gerard A. (1999). The Thermomechanics of Nonlinear Irreversible Behaviors (“science of everything”, pg. 2). World Scientific.
● Tait, Peter G. (1868). Sketch of Thermodynamics (208-pgs). Edinburgh: Edmonston and Douglas.
● Thurston, Robert H. (1878). A History of the Growth of the Steam Engine (Ch. 7: "The Philosophy of the Steam Engine: Energetics and Thermo-Dynamics). New York: D. Appleton and Co.
● Eddy, Henry Turner. (1879). Thermodynamics. Van Nostrand.
● Peabody, Cecil H. (1889). Thermodynamics of the Steam-Engine: and other Heat-Engines. John Wiley & Sons, 1898, fourth edition.
● Cotterill, James Henry. (1890). The Steam Engine Considered as a Thermodynamic Machine (2nd ed.), 426 pgs. London: E. & F. N. Spon.
● Parker, John. (1891). Elementary Thermodynamics. Cambridge University Press.
● Alexander, Peter. (1892). Treatise on Thermodynamics (ch. 3: A Short History of Thermodynamics, pgs. 16-28). Longmans, Green and Co.
● Reeve, Sidney Armor. (1903). The Thermodynamics of Steam Engines. London: The Macmillan Co.
● Goodenough, G.A. (1911). Principles of Thermodynamics. New York: Henry Holt & Co.
● Hartmann, Francis M. (1911). Heat and Thermodynamics. McGraw-Hill.
● Ennis, William D. (1911). Applied Thermodynamics for Engineers. D. Van Nostrand Co.
● McChesney, Malcolm. (1971). Thermodynamics of Electrical Processes. Wiley-Interscience.
● Wallace, Duane C. (1972). Thermodynamics of Crystals. Dover.
● Thermodynamics – Wikipedia.
● Defining thermodynamics (4 articles) – Helium.com.
● Thermodynamics - Zimbio.