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What is a formal rigorous definition of a thermodynamic process?

I went across few references but have not found out a strict formal definition of thermodynamic process.

So any discussion regarding the question might be helpful.

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    $\begingroup$ Considering equilibrium thermodynamics, it's anything that changes any of the state variables of a system(starts from equilibrium - ends at equilibrium) $\endgroup$ – Mann Mar 20 '19 at 14:51
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    $\begingroup$ I am thankful for your effort and time. $\endgroup$ – Bijayan Ray Mar 20 '19 at 15:30
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    $\begingroup$ "A state variable is one of the set of variables that are used to describe the mathematical "state" of a dynamical system." A state variable in case of equilibrium thermodynamics(in case of a homogenous system) is a well defined constant property throughout the system that can be used to describe it. For e.g., the pressure of a system(contrasting with the local pressure) $\endgroup$ – Mann Mar 20 '19 at 15:38
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    $\begingroup$ @Mann Post this as an answer. $\endgroup$ – harshit54 Mar 20 '19 at 15:52
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    $\begingroup$ Going overboard with "rigor" in basic thermodynamics is a classic mistake. The subject doesn't require any more rigor than any other intro physics subject, and it was invented to describe practical things like steam engines. However, because it comes with a lot of new big words ("quasistatic", "adiabatic", "macroscopic", "state variable", "isochoric", "isothermal", "reversible") people treat it pedantically as if it's real analysis. $\endgroup$ – knzhou Mar 20 '19 at 16:26
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Considering equilibrium thermodynamics, it's anything that changes any of the state variables of a system(starts from equilibrium - ends at equilibrium. "A state variable is one of the set of variables that are used to describe the mathematical "state" of a dynamical system." A state variable in case of equilibrium thermodynamics(in case of a homogenous system) is a well defined constant property throughout the system that can be used to describe it. For e.g., the pressure of a system(contrasting with the local pressure).

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    $\begingroup$ I'm surprised you haven't included any reference to heat, which (as a layperson) I'd thought was about half the meaning of "thermodynamics". (On the other hand, neither has either the questioner or any of the comments made to him so far, except for one parenthetical term by knzhou.) $\endgroup$ – Edouard Mar 20 '19 at 17:14
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    $\begingroup$ That's because strictly speaking heat is thermal energy transfer across "the boundary of a system" while the term thermal energy denotes the thermal energy content of a system, we typically shouldn't write "heat content". Now considering that heat transfer is just the phenomenon of transfer of thermal energy across the boundary of a system. You should see why heat is not that fundamental. It's "Energy" (of a system) that is the fundamental subject of thermodynamics, not heat. After that, we can see how systems interact, define the types of process(heat transfer or work done, etc). $\endgroup$ – Mann Mar 20 '19 at 17:25

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