One way to think about it, if we want to use a classical analogy, which often helps to give us an idea of what's going on, is the spin of an electron, we can picture it rotating on its own axis.

如果我们用一个，经典的比喻来考虑它的话，这经常会帮助我们建立起一些概念，就是我们可以把电子的，自旋想象是它绕着轴自转。

麻省理工公开课 - 化学原理课程节选

So you can see, if it's spinning on its own axis in this direction we'd call it spin up, where as this way it would be what we call spin down.

你们可以看到，如果它绕自己的轴朝这个方向转,我们就说它是自旋朝上，这个方向我们叫它自旋朝下。

麻省理工公开课 - 化学原理课程节选