It's related to the heat capacity, the constant volume of heat capacity and something you could measure.

它联系了热能，恒容热容和一些,我们能够测量的物理量。

麻省理工公开课 - 热力学与动力学课程节选

This one turns out to be the heat capacity, and this one turns out to be something that we measure in the Joule-free expansion.

其实，这就是热容，这是焦耳自由膨胀实验中,我们要测量的物理量。

麻省理工公开课 - 热力学与动力学课程节选

We can measure the heat capacity at constant volume, and now we have another term, and if we can figure out how to measure it, we'll have a complete form for this differential du which will enable us to calculate du for any process.

我们能够测量恒定体积下的热容，这里我们有另一项，如果能够知道怎么测量它，问我们就有了这个完整的微分式,就能够对任何过程计算。

麻省理工公开课 - 热力学与动力学课程节选