Thermodynamics starts by drawing a boundary and asking what can cross it
Before physicists can apply the laws of heat and energy to anything, from a car engine to a star, they must decide what counts as the system and what counts as everything else. The walls they imagine around it, and what those walls let through, shape every answer that follows.
A thermodynamic system is a chunk of matter or radiation marked off from its surroundings so the laws of thermodynamics can be applied to it. The boundary may be real or imagined, and its properties decide what passes. An isolated system exchanges neither matter nor energy with the outside; a closed one trades heat and work but no matter; an open one swaps both. Walls can be fixed, like a sealed reactor, or movable, like a piston that can lock in place or slide freely, and they may be idealised as blocking heat, passing it, or letting only some substances through.
Truly isolated systems do not exist, since gravity alone always reaches in. Still, it is taken as an axiom that an isolated system settles eventually into internal equilibrium, where temperatures and pressures even out and nothing changes on a large scale. A metal rod warm at one end illustrates the idea: heat flows until the whole rod shares one temperature. The existence of such equilibrium states is arguably thermodynamics' most basic postulate, with the familiar zeroth law following from it, even though almost nothing in nature is strictly at equilibrium.
Classical thermodynamics works with equilibrium states and the processes linking them. A reversible process would need the system to stay in equilibrium at every step, which is impossible in practice but can be approached by changing things very slowly. Non-equilibrium thermodynamics handles systems with ongoing flows of matter and energy, slow enough to describe with quantities close to the usual state variables. Engineers often treat flowing systems approximately using equilibrium ideas.
Classifications grew with the science, across a span from Sadi Carnot's theory of heat engines in France in 1824 to Ilya Prigogine's work on dissipative structures in 1971. In 2010 Boris Dobroborsky proposed a further split: passive systems that merely redistribute energy, like the cooling rod, and active ones that convert energy between forms, as in chemical reactions, motors or friction, which keep them away from equilibrium.
Source: Thermodynamic system
