孤立系统是与其外界没有任何联系的热力学系统。系统的边界不允许系统与环境进行物质和能量的传递，其物质和能量总量不随时间变化。孤立系统内部压强、温度以及物质分布会均一化。而系统所有的均一化过程最终会使系统达到热力学平衡。

孤立系统在现实中并不存在（唯一的例外可能是宇宙整体），因为系统内部物质和外界物质间会存在像万有引力这样的相互作用。然而，现实中的热力学系统可以在一定时间内处于趋近于孤立的状态。这时孤立系统就可以成为拟合现实世界情况的一个十分有用的模型。而在构造某些现象的数学模型时，它也是一个可以接受的理想化模型。玻尔兹曼在证明H定理（这一定理可以验证熵增加原理）时使用了玻尔兹曼方程。而这一方程只有在系统孤立时才能严格成立。

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