So we can completely describe an orbital with just using three quantum numbers, but we have this fourth quantum number that describes something about the electron that's required for now a complete description of the electron, and that's the idea of spin.

所以我们可以用３个,量子数完全刻画轨道，但我们有这第四个量子数,来完整的,描述电子，这就是自旋的概念。

麻省理工公开课 - 化学原理课程节选