In science, thermodynamics (definitions) are sentence descriptions that explain the subject of thermodynamics. The first definition was stated in 1854 by Irish physicist William Thomson. Other definitions soon followed, at about a yearly rate, soon thereafter; each defined in near synonymous, but subtly different and unique ways; each of which, offering insight into the difficult and complex subject. A chronological table of thermodynamics definitions, stated by various authors, is shown below.

Proto-definitions
See also: Thermo-dynamics (etymology)
It could be argued that early-definitions of thermodynamics, as the science of the relation between heat and work, can be found in the works of those as Sadi Carnot (1824) and Emile Clapeyron (1834). In particular, from the opening page of Carnot’s Reflections, which started the science of thermodynamics, the following definition can be discerned:

“The study of heat-engines [by which] combustibles produce heat and the impelling power which is the result of it.”

In 1845, a twenty-one year old William Thomson went on a search in bookstores throughout Paris looking for a copy of Carnot's Reflections (see: Thomson’s search for Carnot’s Reflections) to no avail; eventually, however, obtaining one a few months later from his professor Lewis Gordon. The influence of Carnot, through Thomson, later functioned to bring the subject and definition into European scientific circles, particularly in Ireland (William Rankine), England (James Maxwell), and Germany (Rudolf Clausius and Hermann Helmholtz).

Thermodynamics definitions table
The following is a definitions chronology table of thermodynamics:

Date
Definitions of thermodynamics
Person



1824
 The study of the principles and laws of the production of motion by heat, considered independent of any mechanism or any particular agency, applicable to not only to steam-engines but to all imaginable heat-engines, whatever the working substance and whatever the method by which it is operated.
Sadi Carnot
1854Thermo-dynamics [has] two divisions, of which the subjects are respectively, the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.William Thomson
1859
 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
1860
 The reduction of the laws according to which the phenomena by which heat, by expanding bodies, is a source of motive power and, conversely, that motive power, being expended either in compressing bodies or in producing friction, is a source of heat, take place, to a physical theory or connected system of principles.
John Nichol
1871
 The investigation of those relations [thermometry and calorimetry] between the thermal and the mechanical properties of substances.
James Maxwell
1880Thermodynamics, 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 Augustus du Bois
1912
 Calculations about heat as a form of energy, and about work, another related form, both of them in connection with changes in the condition of all sorts of substances that may give or take heat, and perform or receive work while changing condition.
Charles Lucke
1936
 [The subject] mainly concerned with the transformations of heat into mechanical work and the opposite transformations of mechanical work into heat.
Enrico Fermi
1941
 The science of the relationship between heat, work, and the properties of systems.
Joseph Keenan
1969
 The study of energy and its transformations.
Hendrick Van Ness
1998
 The science of all transformations of matter and energy.
Pierre Perrot
2001
 The study of energy transformations.
Donald Haynie
2006
 The study of the changes in the state or condition of a substance when changes in its temperature, state of aggregation, or internal energy are important.
Stanley Sandler





References
● Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power” (pg. 6). Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
● Thomson, William. (1854). Thermo-electric Currents, Preliminary 97-101, “Fundamental Principles of General Thermo-dynamics Recapitualted, (pg. 232). Transactions of the Royal Society of Edinburgh, vol. xxi, part I.; read May.
● Rankine, William. (1859). A Manual of the Steam Engine and Other Prime Movers, (chapter III: “Principles of Thermodynamics”, pgs. 299-478). London: Charles Griffin & Co.
● Nichol, John Pringle. (1860). A Cyclopedia of the Physical Sciences: Comprising Acoustics, Astronomy, Dynamics, Electricity, Heat, Hydrodynamics, Magnetism, Philosophy of Mathematics, Meteorology, Optics, Pneumatics, Statics, and etc. (pgs. 411-427). London: Richard Griffin and Co.
● Maxwell, James C. (1871). Theory of Heat (pg. v). New York: Dover.
● 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.
● Lucke, Charles E. (1912). Engineering Thermodynamics (preface). McGraw-Hill.
● Fermi, Enrico. (1936). Thermodynamics (pg. ix). New York: Dover.
● Keenan, Joseph H. (1941). Thermodynamics (pg. 1). New York: John Wiley & Sons, Inc.
● Van Ness, H.C. (1969). Understanding Thermodynamics (pg. 1). New York: Dover.
● Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
● Haynie, Donald. (2001). Biological Thermodynamics (pg. 11). Cambridge: Cambridge University Press.
● Sandler, Stanley I. (2006). Chemical, Biochemical, and Engineering Thermodynamics (pg. 4). Wiley.

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