= Thermodynamic system
A <thermodynamic system> is the part of the physical world selected for an <energy> and matter balance. Draw its boundary before classifying an interaction as <heat>, <work> or transported matter. The boundary may move: a sealed piston encloses a <closed system> even while its <volume (thermodynamics)> changes. An <isolated system> additionally excludes all external <energy> transfers.<Thermodynamics and Propulsion>[Thermodynamics and Propulsion, §1.2.2: system boundaries and closed systems]{ref}
Boundary choice changes the ledger. If an electrical heater warms <water>, choosing only the <water> counts incoming <heat>; including the heater counts electrical <work> entering through its leads. The combined <internal energy> records storage, without separate stored categories for the two transfer mechanisms.<Thermodynamics and Propulsion>[Thermodynamics and Propulsion, §1.3.4, Figure 1.14: resistor and water with alternative system boundaries]{ref}
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