In thermodynamics, a mechanically isolated system is a system that is mechanically constraint to disallow deformations, so that it cannot perform any work on its environment. It may however, exchange heat across the system boundary.

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For a simple system, mechanical isolation is equivalent to a state of constant volume and any process which occurs in such a simple system is said to be isochoric. ^[1]

The opposite of a mechanically isolated system is a mechanically open system,^{[citation needed]} which allows the transfer of mechanical energy. For a simple system, a mechanically open boundary is one that is allowed to move under pressure differences between the two sides of the boundary. At mechanical equilibrium, the pressures on both sides of a mechanically open boundary are equal, but only a mechanically isolating boundary can support pressure differences.

• Closed system
• Thermally isolated system
• Dynamical system
• Open system
• Thermodynamic system
• Isolated system