It's insulated. It's a closed system. In fact, it's an isolated system. There's no energy or matter that can go through that boundary.

系统指的，是盒子里的东西,它是封闭孤立的。

麻省理工公开课 - 热力学与动力学课程节选

It's a closed system, so the number moles doesn't change.

这是一个封闭系统，因此摩尔量不会改变。

麻省理工公开课 - 热力学与动力学课程节选